I don't have many extensions (SimilarWeb, VPN, and Flash Player)

The Flash Player is end of life. Unless it is still useful to you, it should be removed.

Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-ram

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

If you notice you have more content processes than in the past then you may have Fission enabled. Fission is about a new site isolation architecture in Firefox and you will see more Firefox processes running than before and when Fission is enabled you can no longer limit the number of content processes in Settings or via dom.ipc.processCount on the about:config page. You can check the current Fission state on the "Help -> More Troubleshooting Information" (about:support) page (search for Fission).

• https://hacks.mozilla.org/2021/05/introducing-firefox-new-site-isolation-security-architecture/

You can for now disable Fission by setting fission.autostart = false on the about:config page. You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Thanks for your tips, FredMcD.

The Flash Player is end of life. Unless it is still useful to you, it should be removed.

Right, will do.

I've also tried Troubleshooting Mode today for the first time. So far so good. Just to confirm, do I always have to start FF regularly and then basically restart it with the procedure?

I checked the posts you sent right before posting my question, and none of them solved it. That's what frustrated me.

I'll lay hopes in the Troubleshooting Mode, thanks!

If you notice you have more content processes than in the past then you may have Fission enabled.

I read this very article while researching my problem before posting, so I'm guessing there is a connection indeed. I like the added security, but I always close FF (cookies and history wiped out) before AND after visiting my online banking sites, so maybe I'll be safe without?

I'll give that a try as well, thank you.

I've also tried Troubleshooting Mode today for the first time. So far so good. Just to confirm, do I always have to start FF regularly and then basically restart it with the procedure?

https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode

Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes), any userChrome.css/userContent.css files, and some other advanced features to help you assess whether these are causing the problem. If the problem goes away, that tells us one thing. If it does not, that tells us something else.

It could be the work of one of your add-ons, or it could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Type about:addons<enter> in the address box to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All Of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.