Review layout for XUL elements

mozilla bug 1033225 has been switching the XUL layout to use the standard CSS flex layout for elements with -moz-box styling.

This should clean up some weird XUL behaviour. E.g. #41371 (closed). But there may be some initial layout bugs as a result for ESR 115. E.g. the downloads panel used a width: 0 hack to ensure it did not make the panel grow too wide, but with the new layout this makes it wrap every word.

This issue is to go through and check for these kind of layout bugs for the new ESR. But also try and remove some of the CSS hacks that were added for the XUL quirks.