I cannot access various websites in Firefox. Some, like BBC News, will open but don’t display correctly. More worryingly, I cannot sign in to my bank account or my credit card account on Firefox. I am forced to use Edge! I have run various scans with Kaspersky and Malwarebytes and don’t know where to look next.

I have a Dell Inspiron 15 5000 laptop running Windows 10 Home, Firefox 61.0.2 and Kaspersky Internet Security. Angela

All software is corruptible and browsers, being complex, tend to suffer more than most applications. Problems may be due to corrupted user profiles, caches, or badly behaved extensions rather than the browser code, but for the user, the result is the same.

Happily there is no shortage of browsers, and it’s a good idea to keep a few around. Besides Firefox and Microsoft Edge, you should also have a Chromium-based browser such as Google Chrome, Opera or Vivaldi.

All these browsers are written to the same web standards, but the interpretation and implementation of standards can vary. If a site doesn’t look right in one browser, you can always try it in another. Only if it doesn’t work in any browser do you have a major problem.

Upgrade today

As luck would have it, Mozilla released a new version of Firefox on 5 September. There’s a chance that installing a new version of a browser will fix the problems with the old version, so try installing version 62.0 before trying Refresh Firefox (below).

The latest Firefox doesn’t appear to include any significant changes. However, it’s a good idea to keep all browsers up to date because it reduces the security risk.

Check for malware

You’ve already checked for malware, which is good, but unlikely to help in your case. Browser malware usually shows its presence by throwing up advertising pop-ups, browser redirects, changing the home page and similar actions, rather than through broken web pages.

However, if you suspect malware is involved, run these three programs in order: Microsoft’s Safety Scanner, Malwarebytes, and Hitman Pro. If one of them finds malware, run them all again.

It’s also worth running Qualys’s BrowserCheck online. Installing the BrowserCheck plugin takes longer but provides a more thorough scan

Refresh Firefox

Problems in all browsers are tackled in much the same way. However, Firefox offers what can often be a quick fix via its Refresh Firefox page.

Clicking the “Refresh Firefox” button online restores Firefox to “its default state while saving your essential information like bookmarks, passwords, and open tabs”. It also removes extensions and their stored data.

Firefox keeps all your bookmarks, passwords, history, site-specific preferences and other data in a profile folder. “Refresh Firefox” creates a new profile folder and copies essential data from the old folder to the new one. However, your old profile is not lost forever. Firefox leaves it on your desktop in a folder called Old Firefox Data, in case you need to rescue something important. The drawback is that the more of the old data you recover, the more likely you are to restore whatever caused the problem in the first place.

In my experience, Refresh Firefox is the easiest way to get Firefox working correctly again.

Fixing extensions

Browsers tend to develop in the same way as operating systems, office suites, processors and even computers. The current products always lack features that some people need, which creates an aftermarket for add-ons and enhancements. The most popular enhancements then get integrated into future versions of the product.

Today there are thousands of add-ons, extensions and themes for all the leading browsers except Edge. You could be running a dozen or more. Sadly, extensions often cause problems because of programming errors, or because of unexpected conflicts with code on websites.

The best way to isolate these problems is to disable or uninstall all the add-ons, extensions and themes and see if the browser works correctly without them. If it does, add them back one at a time until you find the one that is causing the problem.

Some browsers include a “safe mode” that is similar but unrelated to the safe mode in Microsoft Windows. If you run the browser in its safe mode (in normal Windows), then it should load without any add-ons and extensions. It may also change other settings temporarily.

To restart Firefox in safe mode, go to the “hamburger” menu button (three horizontal lines) and click Help. From the resulting menu, select “Restart with Add-ons Disabled …”. Another way to do it is to hold down the Shift key while starting Firefox. When you have finished, simply restart Firefox in the normal way.

Mozilla also has a useful help page: Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

Google Chrome doesn’t have a safe mode, but starting it in incognito mode – the private browsing mode – does much the same thing. If Chrome will open correctly, access incognito use the keyboard shortcut Ctrl + Shift + N, or click on the three vertical dots in the top right hand corner to access the menu and click on “New incognito window”.

If Chrome won’t open correctly, copy and paste the Chrome icon on your desktop to create a new shortcut. Right-click the new icon and select Properties. Go to the Target line and add “–incognito” to the command line (without speech marks), then click OK. Double-click your new icon and Chrome will run in incognito mode without any extensions loaded. (If you want to run an extension in incognito mode, go to the Extensions page, click Details, and select “Allow in incognito”.) Delete the extra Chrome icon when you have finished with it.

It might be easier to turn off Chrome extensions manually. To do this, go to its truncated hamburger menu (three dots), select More Tools and then Extensions. Try pressing the Update button (which is only visible in Developer mode) to update all your extensions first, to see if this fixes the problem. If not, turn them all off then add them back one at a time.

Clearing the cache

When you visit a website, your browser adds the address to its history and saves copies of pages in its cache. In Internet Explorer’s day, these were known as Temporary Internet Files.

When you revisit a website, your browser will reload the stored pages, because this is quicker than fetching them all from the internet again. If the cache is corrupted, you may see a corrupted page.

You can force any browser to fetch a fresh version of a page by pressing Ctrl+ F5. This might solve problems with sites like BBC News.

If the problem is more extensive, try deleting the whole cache and, perhaps, cookies as well.

In Firefox, click on the hamburger menu, select Library, then History, and then “Clear Recent History”. This pops up a box that lets you select how much you want to clear, from Last Hour to Everything. Click the Details button for a list of things you can clear, from browsing history to site preferences. To begin with, try deleting just the cache. If that doesn’t help, delete the cookies, and so on.

In Chrome, click on the hamburger menu, select More Tools and then “Clear browsing data”. As with Firefox, you can pick a time range and which elements to clear. Clicking “Advanced” lets you clear things you’d normally keep, such as passwords, media licences and autofill form data.

Reinstall the browser

Removing malware, disabling extensions and clearing caches usually solves most browser problems. If not, there’s the nuclear option: uninstall the browser, restart your PC, then do a clean installation.

You will, of course, lose all the passwords, site preferences, browsing history, bookmarks and everything else you haven’t saved, so it should be a last resort.

Have you got a question? Email it to Ask.Jack@theguardian.com