JO+, I don't have a German or Swedish keyboard to try this on, but if you're running off of a liveCD and could try this it might work. I looked at the use of xmodmap here and I think it should work. I thought that the problem was that firefox grabs the <gr> key and uses it as another alt key. Does it help you to make the following .Xmodmap file
Code Sample
remove mod4 = Mode_switch add mod5 = Mode_switch
and then run xmodmap .Xmodmap
Could someone with one of those crazy keyboards try this and see if it helps firefox?
EDIT: added spaces per JO+'s observationyes, clacker, that's it - great!
I confirm this works with a crazy german keyboard and both live CDs DSL-1.1.iso and the new DSL-1.2.1.iso. xmodmap complains, if you don't use the exact syntax, requiring spaces around the '=' :