I switched Firefox for Chromium when I saw how extremely slow Firefox on Linux is.
Chromium is "the open-source projects behind the Google Chrome browser", supposedly without Google's bad "spyware". I'm also using the LastPass extension and I'm not having any issues.
On the otherhand I also didn't have any issues with Firefox and LastPass on Debian.
Where do you get your Firefox packages? Have you tried https://mozilla.debian.net/ yet?