Thanks for your detailed issue.

Have you try starting Firefox from command line a little and check for error messages in the terminal?

You can also have a look at Performance and Memory in Web developper tools (Ctrl + Shift + i) There a lot of info.

I also have an old computer (10+ years) with Linux but do not suffer performances problems even with 100+ tabs and mutliples addons.

How many CPU and RAM do you have? Strange that it only begin some weeks ago.

Do you have x11 or Wayland ? echo $XDG_SESSION_TYPE

I am not sure when it started. It could have been when I dist upgraded twice from 20.04 to 24.04. I did have a number of compatibility issues due to broken packages, but I think they are all fixed. I don't specifically remember which came first or if it happened because of this.

I am on x11

No error on terminal when starting.

In the dev console I actually errors without opening any tab:

   Content-Security-Policy: The page’s settings blocked an event handler (script-src-attr) from being executed because it violates the following directive: “script-src resource: chrome:”
   Source: function() {
       [native code]
       } 3 home
   Layout was forced before the page was fully loaded. If stylesheets are not yet loaded this may cause a flash of unstyled content. react-transition-group.js:9:10154
   Source map error: Error: URL constructor:  is not a valid URL.
   Stack in the worker:resolveSourceMapURL@resource://devtools/client/shared/source-map-loader/utils/fetchSourceMap.js:56:22
   getOriginalURLs@resource://devtools/client/shared/source-map-loader/source-map.js:73:24
   workerHandler/</<@resource://devtools/client/shared/worker-utils.js:115:52
   workerHandler/<@resource://devtools/client/shared/worker-utils.js:113:13
   
   Resource URL: about:home
   Source Map URL:
</pre>


I tried running the profiler. I am not sure what I am looking for. It seemed that most of the usage in "Privileged cont" was display list building.
After railing the CPU for about 5 mins the CPU usage seems to have dropped back to almost nil. This didn't happen with my old profile. I didn't at any point add any extensions back, not actually start browsing.

Can post similar info from kinfocenter? Also post a screenshot showing Firefox using the cpu. see screenshots Here are pic of my system with several windows from different browsers and several tabs in each window. Test your OS via live usb stick and see if you have the same issue What desktop?

Operating System: openSUSE Tumbleweed 20250321 KDE Plasma Version: 6.3.3 KDE Frameworks Version: 6.12.0 Qt Version: 6.8.2 Kernel Version: 6.13.7-1-default (64-bit) Graphics Platform: X11 Processors: 16 × AMD Ryzen 7 5825U with Radeon Graphics Memory: 62.1 GiB of RAM Graphics Processor: AMD Radeon Graphics Manufacturer: HP Product Name: HP ProBook 455 15.6 inch G9 Notebook PC

kinfocenter:

Operating System: Ubuntu 24.04 KDE Plasma Version: 5.27.11 KDE Frameworks Version: 5.115.0 Qt Version: 5.15.13 Kernel Version: 6.8.0-55-generic (64-bit) Graphics Platform: X11 Processors: 4 × Intel® Core™ i7-7500U CPU @ 2.70GHz Memory: 15.4 GiB of RAM Graphics Processor: Mesa Intel® HD Graphics 620 Manufacturer: LENOVO Product Name: 20JH003BUK System Version: ThinkPad Yoga 370

Update So, this is one of the least helpful "it seems to be fixed" updates. But, this is what has happened.

I tried to get a longer set of memory data yesterday, still with no tabs. And at somepoint (after about 5 mins the CPU usage dropped back to normal).

I have since, added back in Ublock Origin, and Firefox containers. And have been using the browser like normal for about 24 hours without the same issues. This is odd as the old profile did it for weeks without stopping.

I have not returned to my old profile to see if usage spikes back up, as I am very much in "ain't broke don't fix" mode as this is my main work machine.

I can confirm that if I close and re-open firefox it doesn't do the same thing. It very quickly drops to a normal load.

It seems like firefox may have been trying to build some sort of index, and when I create an entirely new `.mozilla` dir it needed to do this and it took 5-10 mins, but I kept being impatient and closing it. On my old profile perhaps something (that is not cleared by firefox refresh) is broken and this indexing never completes? I say this with almost no knowledge of how firefox works, so I may be completely wrong.

Anyway, I'm not sure what to do now. I can try the live USB, or opening my old profile and see if the problem returns. But that will probably have to wait for next month as I have some deadlines approaching and I am just delighted to have a working browser again on my main machine. Thanks for your help and sorry that I can't provide a definitive cause of what fixed it.