Ah, interesting. Well, if you create a bug report, post the link here and I’ll vote it up for visibility :)

I believe it’s because firefox’s UI is placed on the bottom of the screen per default. To account for that, I believe they adjust the viewport’s height to either exclude the bottom of the screen (when the symbol bar is displayed) or include it (when you scroll down and the symbol bar is hidden).

Because they use a slide-out animation based on scrolling within the web page, the viewport changes a million times in height and causes elements fixed to the bottom of the viewport to jump and adjust a lot, causing weird behavior. And nobody tests for that.

All of thse are my assumptions by the way, please test for yourself, I might well be wrong.

If it really bothers you, try placing the symbol bar above and see if it works. It’s in the settings.