In my Firefox, according to the Task Manager in about:performance, the LanguageTool add-on is consistently among the most resource-intensive ones, and typically is the top one. That’s something, given that I have over 100 add-ons active.
This could be maybe understandable while I’m typing in Firefox, but this happens even when the browser is idling in the background!
When I noticed this in the past I just disabled LT, thinking I’d re-enable it as needed, but then of course forgot about it for months if not years. Now I rediscovered it, again liked it, … and once again am about to disable it because of resource usage.
So I thought I’d leave a note somewhere in case this can be fixed in some future version.
This is in Firefox 115b7 on macOS 13.4. But as commented, this has been happening for a long time.