Another thing that came to mind.... You can try to reset the zoom. Set browser.display.os-zoom-behavior to 0 and then back to 1. You might have to adjust your prefered zoom again using ctrl+mouse wheel up or down after this for each page. But firefox...
I have been thinking here... The ui.textScaleFactor is a fix for the font size in the tab text and bookmark text size. Your gmail screen is huge correct ? Try zooming it out until it's on the size you are used to. Firefox should memorize the zoom siz...
