I am a (reasonably) new web developer. As I've been getting more into testing / bug finding, I keep finding things that don't work on Firefox, exclusively. Often, when I've found a solution to one of my problems on SE, I find there's some sort of caveat or exception made for Firefox. In my GitHub (for a personal project), a fair fraction of problems (currently a majority) are tied to how Firefox renders the pages / elements.
My questions are: