could you go to the firefox menu ≡ > help ? > troubleshooting information, copy the contents of that page and paste them here into a reply on the forum (or if it's too long through a site like https://pastebin.mozilla.org/ and provide the link here)?

this might give us a clue what is going on. thank you!

Me too, in spades: https://pastebin.mozilla.org/9074260

I too have been having an issue with the new Firefox UI. For me, anything that seems to generate an overlay on the UI (e.g. the options from the hamburger menu, the bookmarks button, add-ons being installed, etc.) seems to cause the application to become unresponsive while the item tries to load - resulting in about a 50-50 chance that it will finally show up after about 30sec to a minute. I have tried setting the accessibility.forec_disabled to 1 and also starting in Safe Mode has no effect for me. This also happens on my local machine (a Windows 7 PC) and a Windows 8.1 VM (which is hosted elsewhere with my work). I have uploaded the troubleshooting information to https://pastebin.mozilla.org/9074481. Please let me know if there's anything else I can to to help, thanks!