I just passed through one of those Linux episodes of "be careful what you install", caused as always due to deficient dependency management. Well, to be fare, I can't blame my distribution for breaking applications, because the broken apps where 3rd party installations (namely mozilla products). And all because I wanted to finally install hangul (Korean language characters) support in my Kubuntu box.
Lesson learned: Don't install scim and uim packages if you are going to use Firefox, Thunderbird and Adobe Acrobat reader. Apparently there is conflict with binaries compiled with different branches of the glibc library. There are some workarounds, but since I'm not sure about all the programs that are using that library, I'd rather uninstall the whole thing that having some problem in a couple of weeks once I forget the issue. I might try those solutions some other time. For now, I had enough time wasted finding out what was the problem (for which, by the way, was rather handy to have Opera installed when I couldn't run Firefox)