I am just putting this here so that this might show up a bit easier in a Google search. I recently figured out a solution (sort of) to one of these nagging issues that I have been facing for a long time. The issue is that sometimes Firefox gets in a state where it hangs for a few seconds, pegging the CPU, whenever the window comes into focus (i.e. you click on it). This means it is very difficult to switch back and forth between Firefox and another window. This was a mystery for a long while. However, I found these two bug reports today. The earlier bug was reported around two years ago, and they supposedly pushed a fix around 8 months ago, but it is still biting (me) today.
This seems to actually be a limitation of the current implementation of the global menu in Ubuntu. This works via DBUS, which apparently isn't very efficient at populating large menus. Firefox has a bookmarks menu which means that having many entries in your bookmark database will cause Firefox to hang as that menu is populated. This was actually pretty difficult to figure out as the behavior is intermittent. Often Firefox would work as expected, but sometimes this hanging would start and it would persist until reboot of the entire system (restarting Firefox is not enough). I now suspect that this state is triggered by setting a bookmark, but I am unsure.
Anyway, the fix here is to not use bookmarks too heavily. If you delete all of your bookmarks this problem will go away, and this is just what I did (after I saved them to disk). That said, this really puts me, and people who browse the Web like I do, in a predicament.
When I find an article or web page that I am interested in, I cannot usually read it when I find it. So, I like to store it for future consumption. For this purpose, I used to use tabs, but as everybody knows, this will bog your computer down as most browsers attempt to keep them in RAM. Firefox, fairly recently, started releasing them from memory after they haven't been used, which helps matters (unfortunately they made the, IMHO, stupid decision to completely reload on restore rather than cache to disk). But, even with this memory friendly behavior from Firefox, it will still routinely bloat to greater than 1GB of resident RAM and needs to be restarted (presumably so that the tabs will be flushed out of RAM). And before the "Firefox is just making efficient use of memory" people show up, Firefox is not making good use of memory if it causes my computer to hang for 30+ seconds when switching windows that have nothing to do with Firefox (which it does in certain extreme cases), or if it causes Compiz, Xorg, or my entire computer to crash (which happens every once in a while and correlates so strongly with Firefox memory consumption that I have to suspect it as a culprit).
To combat this, a while ago I started categorizing pages I find as I want to read this within a week or I want to read sometime later in the future. The ones I want to read within a week I keep as tabs and the ones that I want to read sometime in the future I set a bookmark to find it easily later. I figured that bookmarks should take practically no resources to hold other than a bit of disk space. With this bug, however, setting bookmarks on interesting pages isn't really a good option either. This means that I now have my bookmarks in a separate HTML and JSON file while I try to come up with a better solution.
I suppose that this isn't an issue with Chromium or Chrome as those don't use the global menu for bookmarks (they have a separate menu right of the address bar).