from 2017-09-03:
My Firefox was bogged down for at least a year, but ever since I created a new profile, it's super speedy! Sometimes I have to quit Firefox and reload the old profile (which is painfully slow) to grab a bookmark, but other than that, it's been great to have Firefox be more responsive again. Follow the instructions at https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles#w_starting-the-profile-manager to read about managing multiple profiles.
At some point, on Mac, Firefox's "highlight all" hotkey changed from ctrl-a to cmd-enter and cmd-L.
Firefox can be made to "highlight all" by default when searching: https://superuser.com/a/642824/329095