You could try a newer Firefox version, like beta or the Firefox nightly, and see if Mozilla is working on fixing it. I’ve noticed some theme bugs (scroll bars were too wide) are fixed now in Firefox 52, which is currently in beta. [Edited to add:] The way Firefox’s versions work is that beta is one version ahead (so 52 now) Developer Edition is two versions ahead (so 53 now), and nightly is three versions ahead (so 54 now).
There’s an official PPA for Firefox beta: https://launchpad.net/~mozillateam/+archive/ubuntu/firefox-next. It replaces .the regular Firefox, but I’ve used it for months without any issues. (Before that I used Firefox beta on Mac for years also without any problems personally.)
You can also download Mozilla’s binary releases (beta; Developer Edition, which is their alpha; or Nightly: https://www.mozilla.org/en-US/firefox/channel/desktop/. These don’t install, so they don’t replace anything, you just unpack them and run the firefox
binary.
Note that Developer Edition uses a new profile by default while beta and nightly use the same profile as regular Firefox on the same machine. In theory using a newer version could make your profile incompatible with the older version after that, but I switch between beta and nightly constantly and have never had a problem.