The most annoying thing I’ve had to deal with in Firefox 1.5 is a Linux-only bug in which the first time you do something in a certain class of actions—drag-and-drop, Save As…, and a couple of other things—it will lock up for a period of time (1 second to 30+ seconds, depending on how many tabs & windows you have open), scroll all pages back to the top, and in some cases, bring up an unclickable extra copy of the pop-up menu.
The bug was fixed before Firefox 1.5 was released, but it was too late for the fix to make it in. I actually suspect this bug is one of the reasons that so few Linux distributions have upgraded their built-in Firefox releases to 1.5, though they could always have just added the patch to their build process. Fortunately, Mozilla has decided that it’s worth including in the first bugfix release in the 1.5 series, which is now available for testing. Firefox 22.214.171.124 should be out sometime in the next few weeks.