3.0.621 does not uninstall properly?

I have tried 3.0.621 on computer where previously was installed 2.7.

621 was working properly and after test I have uninstalled it.
After reverting back to 2.7 I find that firefox have issues with Java, which I never noticed before - it will randomly shut down, if several tabs with Java open and closing one tab.
Installing back 3.0.621 solved the problem, but it is not a solution.

Are there any cleaning tools or instructions available, or something is damaged at driver level?

 

I would like to know this also. I had installed a beta of version 3 when the beta came out. I uninstalled it and things didn't work correctly after that especially with any firewall id install