Firefox 3.6.8. messed up an older version of Firefox

Question

As the title indicates, I believe that installing Firefox 3.6.8. messed up my favorite version of Firefox, version 1.5.0.3, or something like that on Windows XP SP3. I installed 3.6.8. to another folder, like I did a while back with 3.0.1, which is still on my computer, and thought it would function just the same and not enable my older version useless, but it seemingly has done so. I did not move any Firefox related folders prior to installation. Assuming you can see the picture that will hopefully work when viewed, you can see there's what looks to be an error message on the bottom. Also, you might notice that there is a lot of stuff missing, like the 'Go' button, search field, that circle with 8 smaller circles, and also all of the other stuff that should be there in the about:config. It's like it set itself for a larger resolution or something and I just can't get to what would normally be on the right side. The window doesn't get much larger than when its set at maximized. So almost all websites are not viewable because everything the site has is off screen to the right.
I don't know what to do at this point, I tried searching for info but it's hard trying to figure out what exactly to search for. I uninstalled both versions, moved both mozilla folders in the application data folders to somewhere else, and then reinstalled my favorite version but the problem was still the same. Luckily I kept the old folders otherwise I would've lost my extensions(called add-ons now) and bookmarks. I'm aware that the obvious option is to just use the newer ones, but I've grown accustomed to that version and I like how simple it looks. I'm hoping it's an easy fix, haha. If it is a matter of my version being just too old and nobody knows anything about it anymore, then I'm fine with that. But, if someone could help me out here I'd greatly appreciate it.
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Answer 1

Thank you for taking the time out to reply. I don't know whether or not your suggestions would have worked, but I did solve my problem. I eventually figured out that the file, overlay.xul, was doing something and when I changed the filename, the bar went away.
I was reading that the xul error usually meant something wrong with an extension, but that didn't make sense because after uninstalling/reinstalling, there were no extensions present, on an assumption, of course. I noticed a file through a search, and it was located in a folder without a mozilla/firefox name in it, looked like a randomly generated folder name. I looked in the extensions. ini and noticed it pointed to that overlay.xul file in that random named folder and thought, what the hell, and just renamed the overlay.xul file to something else and the problem went away.
Thanks again.