(In reply to Catalin Sasca, QA [:csasca] from comment #6)
I successfully reproduced the issue on Firefox 81.0a1 (2020-08-06) under macOS 11 by selecting all the text from this page and activate the speak selection feature.
The hangs are no longer reproducible on Firefox Nightly 81.0a1 (2020-08-16), but I couldn't use the text selection feature on 80.0b8. It simply won't activate by using the key combinations.
Any idea what could have happened here Eitan?
The patch in this bug temporarily deactivated the speak selection feature until we could figure out a performant way to do it. It will remain disabled in 80, but I recently landed a patch that makes it fast again in 81, and re-exposed the API in bug 1659215. So now this is fixed and should not be reproducible in nightly.