My biggest gripe with Firefox is how it opens tabs. If I open a new tab in Chrome, it opens just to the right of the current one which is nice because it keeps related tabs together. In Firefox it opens to the far right which is annoying. I tried installing an extension to fix this but it was buggy. Only recently did I discover that I could dig into about:config to enable the behavior I consider expected (and I'm sure most people agree with me given how many Chrome users there are).