I’ve been in contact with the LastPass team regarding an issue where the LastPass toolbar pop-up is completely blacked out and the text is not readable in Firefox.
After some troubleshooting, what I found what that it’s related to the Ubuntu theme.
I was able to reproduce the issue using both Ambiance and Radiance, but the issue went away changing to any other theme.
Themes are accessible by going to System > Preferences > Appearance.
UPDATE: It seems that this is specifically related to the tooltip background color.
Going to Theme > Customize > Colors and changing the tooltip background color solves it.