Firefox is too slow lately

[Eduardo1]

Since a few weeks ago I noticed that Firefox makes my processor fan to run, then I checked with the Windows Task Manager how much processor it uses, and it uses like 30% (total, of all my 6 cores) having only one tab of Youtube open with a live stream video.

I tried with the troubleshoot mode with all extensions disabled and it was the same.

What happened in latest FF updates, that ruined FF so much?

[tate]

Many users have recently reported similar issues, with CPU usage jumping to around 30% or more even with all extensions disabled and Firefox running in Troubleshoot Mode. One of the most common culprits is YouTube’s Ambient Mode, which adds a background glow effect around the video when using dark theme. While visually appealing, this feature can significantly increase CPU and GPU usage. Turning it off often results in an immediate performance boost. Another major factor is video codec handling. Firefox sometimes uses high-demand codecs like AV1 or VP9 for video playback, which aren't always hardware-accelerated especially on older hardware. To resolve this, it’s recommended to enable hardware acceleration in Firefox settings and disable AV1 support by visiting about:config and setting media.av1.enabled to false. You can also install extensions like h264ify or Enhancer for YouTube to force the browser to use less demanding video app formats like H.264, which are easier on your CPU.

[Eduardo1]

OK, thank you. I had both circumstances you described. I made both changes. Not let's see. 

PS: still, something changed lately.

[Eduardo1]

The issue is not solved. It seems to be a bit random, because "sometimes" it doesn't use more than 2-3% (most of the time it uses over 20%).

What else to try?

[IanC]

I tried those and it didn't improve. Now 1080p videos are unwatchable, and I have to drop to 720 or 480. Something changed in the most recent update. Other browsers work fine.

[dada]

Yes, me too. Firefox version 139.0.4 (64-Bit) is very slow and crashed today (June 23, 2025) already 3 times.
Extremely annoying :-(((

[Agentvirtuel]

Hello

Task Manager - see whether tabs or extensions are slowing down Firefox
https://support.mozilla.org/en-US/kb/task-manager-tabs-or-extensions-are-slowing-firefox
Perform actions in the Process Manager

Maybe
Reporting a Performance Problem
https://firefox-source-docs.mozilla.org/performance/reporting_a_performance_problem.html
You can add the profile URL to a bug.

[Eduardo1]

I have noticed that the live streams videos at Youtube use a lot more processor than regular videos. Regular videos:  ~10%, live streams: ~40%.