Had this same problem for weeks on my work machine. Firefox’s process manager shows you exactly which processes are eating your CPU - that’s where I’d start.
The AI sidebar and translation features killed my performance. Disabled the sidebar completely and went back to the Google Translate extension. CPU usage went from 40% idle down to 8%.
Here’s what really helped - Firefox now caches tons of AI model data locally. Clear that cache and you’ll see a big difference. Go to about:support and wipe “Cached Web Content” and “Offline Web Content”.
Windows users: check if Firefox is running background tasks when it’s closed. The AI stuff pre-loads and keeps running. Found three new scheduled tasks in Task Scheduler after the update that I didn’t need.
Worth spending 30 minutes tweaking these settings instead of switching browsers. Mozilla usually fixes performance issues within a month of major updates anyway.