thank you for the fast replays! before i go and test your suggestion i will post some more info about this, maybe someone allredy knows about the problem and has a solution...
before mandriva 2005LE i had mandrake 10.0. in that version there was no firefox package, so i had to download and install the installer from mozilla. at the begining i had the same isue, i was not able to run it in user. the solution was to remove the firefox (delete the mozilla folder), reinstall it and then to run it in root for the first time. it seems that this creates "valid" directories and files. then i was also able to run it as user.
the problem this time is that i have install it from the mandriva install center and since i am a bit new to linux i dont know what to do since i dont know where firefox is installed this time (since i was using the madriva install center).
i can add that i have tried to remove it using the mandriva uninstall center, and then reinstall it and run in as root the first time. but this didnt help (i guess i shoudl find the firefox profile folder after the uninstall and delete it before instaling the firefox again).
and it is also a bit strange that the instalation of the firefox from mandriva instalation center does not work out of the box