Very Strange problem.. I Cant understand why..

I was fiddiling around on my server trying to find a good plugin for displaying video from my cell phone. While i was doing that i was adding and delete’n alot.. but only in the plugin dir.
But efter a while I gave up. But to my horror when i cheked back at my blog the whole background for admin had disappeared It was just Text and numbers, and some small gfx , but mostly it was just fyckup up. And my own header gfx I had put in there was gone.

So I was like.. sigh.. lets remove the whole thing and unpack the backup i made 2 days ago. So i did that… but when i tried again, IT WAS JUST LIKE BEFORE!?! The blogg looks normal to the outside viewer but not to the admin.
And then i discovered another thing, I tried to access the page in both Opera and IE. And in both of them it looks normal, even the admin page.
Why The H-LL does it not work with firefox???? Ive always run firefox and never had a problem with it.

PS im running Classic theme.

Can someone plz helt me out, this is really annoying.
(and while ur at it.. give me a good a versatile plugin for running cellphone videos on yer site, and that lets u store ur videofiles on my site)