Hmm, I don't know why that is happening.

Is this limited to add-on drop-panels? For example, it doesn't affect the Add Bookmark panel (click the star icon) or Downloads panel (click the arrow icon)?

Since you mentioned that you have multiple displays, do they have different resolutions? If so, do you notice the problem occurs after you move the Firefox window from one display to the other?

Hi there, thanks for replying.

Yes it's limited only to 3rd party plugins. All the Firefox native ones work fine.

Also yes, my displays have different resolutions. It actually works fine on the "native" 4K resolution, but not on either of the 1080p screens.

Kind regards, Nick

Hi Nick, thank you for the additional details.

It might be useful to test a future version of Firefox to see whether a fix is already set to come, or whether this is an unsolved problem. To avoid mixing up your settings, you could use a "Portable" build, which installs into a single folder. You then would need to add at least one of those extensions for testing.

Firefox 56 in Beta https://portableapps.com/apps/internet/firefox_portable/test

Firefox 57 in Nightly https://portableapps.com/apps/internet/firefox-portable-nightly

If the problem still exists in Nightly, you can submit a new bug report here:

https://bugzilla.mozilla.org/enter_bug.cgi

You can try to disable hardware acceleration in Firefox.

• Options/Preferences -> General: Performance
  remove checkmark: [ ] "Use recommended performance settings"
  "Use hardware acceleration when available"

You need to close and restart Firefox after toggling this setting.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

@core-el - thank you for the suggestion. This unfortunately did not work.