An updated occured to my Firefox recently and yesterday I noticed that every time I booted Firefox, it will have a certain size and position regardless of resizing the window or moving it.  Seemed as though it was no longer saving the window size and placement.

After a bit of research, I found that it was due to a corrupted LocalStore.rdf file.

http://kb.mozillazine.org/Search_engines_disappear_from_Search_Bar

The page above gives a method of fixing the problem, but for me, with Firefox closed, I simply renamed the file "localstore.rdf" (located in your ApplicationData/Mozilla/FireFox/(some profile name)/localstoer.rdf) to something else and then rebooted FireFox.  No problems at all.

BTW, if you get tired of always fillling out forms (such as name, address, phone number, etc), check out the extention called "autofill".  They now have a version that works with the latest Firefox.