Maybe check for memory issues.

You can check the Firefox Task Manager (about:performance) and about:memory pages.

• https://support.mozilla.org/en-US/kb/task-manager-tabs-or-extensions-are-slowing-firefox
• https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-or-cpu-resources

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

I see your previous question here: https://support.mozilla.org/questions/1296747 I think that is just a really difficult question to answer.

Thank you very much for offering a comment on that question. This comment surprises me a lot. I figured this behavior must be well known and there must be some kind of well-known cause and an explanation of why it has not been resolved. I have noticed this behavior (browser slowing down after hours of heavy use) for at least five years.

For what it is worth, way back in the day, browsers, possibly including FireFox, had a habit of increasing memory usage when used for long periods, causing slowdowns. That problem was attributed to memory leaks, but I cannot verify that was the actual cause. I can verify the problem would be cleared by restarting the browser. My perception is memory leaks cannot be the cause now, because QA of major software such as FireFox would prevent that from happening. I am guessing the problem is associated with cache. Perhaps I have a bad combination of browser configuration settings causing this.

I have been having the same issue I think. Can't play your demo link. After awhile Firefox becomes extremely laggy and I have to restart it. I've removed all my extensions except for ublock and lastpass and that didn't help. I'm going to try and disable my theme and see what happens. I'll look at the task manager (as indicated by cor-el) next time.

Well, no luck it still gets laggy. Task manager doesn't show anything abnormal that I can see.

whenever I load a website firefox becomes too much slow I'm annoyed with it. I'm thinking to change my browser right now, even my internet connection is stable.

Similar problem here. Disabling some or all add-ons improves it somewhat, but within a couple of hours its back slow as snail, then it just starts working fine again, or restarting it will return it to normal.

The most obvious problem is when typing, it just slows down to the point of being useless. Then its back to normal. This problem seems to have gotten worse with each of the last few updates.

What else can I look for that could be the problem?

I have a laptop with 4GB of RAM and an old magnetic hard drive. After some browsing, FF becomes slow and the hard drive is mostly on. The Task Manager then shows that most RAM is in use by FF with only a few hundred MB of free memory remaining.

I never have slowness on PCs with 8GB of RAM or more and a fast SSD hard drive. Hence, I believe the slowness is from the hard drive paging memory in and out.

Check the free memory right after you start FF and again when it becomes slow.