Start your Computer in safe mode with networking. Then start Firefox. Try Safe web sites. Is the problem still there?

Starting The Computer In Safe Mode;
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While still in Safe Mode,

Type about:support in the address bar and press Enter. Under the main banner, press the button; Copy Text To Clipboard.. Then in the reply box at the bottom of this page, do a right click in the box and select Paste. This will show us your system details. No Personal Information Is Collected.

https://support.mozilla.org/en-US/kb/websites-look-wrong-or-appear-differently

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-hangs-or-not-responding

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

I, like many *MANY* others, have similar issues with FF on a Mac. Even in safe mode, my CPU usage is over 100%, at times higher than 160%. Consistently using a gig of RAM, or more. Slow, unresponsive. After many years of this problem, which only gets worse, I've reached a point where I am going to drop FF in favor of Chrome.

There are clearly deep-seated problems with the FF code that Mozilla doesn't want to admit to. Instead, we are given a bunch of links to crappy solutions that don't work, which all of us have tried multiple times. Such behavior is insulting: most of us, in the simple of act of posting on this forum, have already demonstrated familiarity with computers and programs that far exceeds the n00b approaches presented in the FAQs.

The code stinks, but that would almost be acceptable if Mozilla would tell us what they are doing to address it (as far as we can tell, nothing), or where they think the problems may lay. However, they have chosen to not even acknowledge the problem, so it is off to much greener pastures for me, where my computer doesn't act as a space heater because of its ridiculous CPU usage.

This has been an ongoing problem for at least two years for me. It's the same problem as many users have described in this thread, which unfortunately was archived.

Firefox memory and CPU usage steadily keeps growing throughout the day until I need to restart the browser to be able to do anything useful with it. The solution is not Safe Mode or to disable add-ons, I have tried all of this countless times. At the very moment I run the browser without any add-ons.

The workaround I found after two years of wondering, with no useful solutions being offered, was to DELETE my Firefox profile folder inside *username*\AppData\Local\Mozilla\. Note that this only deletes temporary files and image cache. Browsing history, passwords etc. are stored in *username*\AppData\Roaming\Mozilla\, so no important data is lost. With periodically deleting above mentioned folder I can now manage to run Firefox for almost a week without insane CPU and memory use developing.

The other solution I found was to reboot my entire OS. After a full reboot I can again use the browser for a couple of days without issues (note that just restarting the browser itself seems not to be enough).

These are the two solutions that should be advertised here instead of the obvious Safe Mode that is brought as a "solution" to every problem.

Well, recently i was suffering this same problem. My CPU was 100% but both RAM and Processor was fine. My OS is Windows 8. So here's what i've done to solve the problem:

1. Stopped it's process from "Task Manager".

2. Uninstalled Firefox browser.

3. Deleted it's directory "Mozilla Firefox" from "C:\Program Files".

4. Deleted the directory "Mozilla" from "C:\Users\*username*\AppData\Local". [it took couple of minutes to complete the deletion process and during the process my HDD was 100%. So i waited and it worked at last.]

5. Deleted the directory "Mozilla" from "C:\Users\*username*\AppData\Roaming". [it took couple of minutes to complete the deletion process and during the process my HDD was 100%. So i waited and it worked at last.]

6. Restarted the PC.

7. Cleaned ("temp", "%temp%", "prefetch") through "Right click on start mentu > Run".

8. Cleaned other browsing history and junks in "Internet Options" from "Settings".

9. Run CCleaner and cleaned the PC (Check and uncheck your options). [it took couple of minutes to complete the process and during the process my HDD was 100%. So i waited and it worked at last.]

10. Download and re-install Firefox again.

Now, it's working like a charm!

Thanks, sazu489.