From what you say, I deduce that you didn't use a Ubuntu rpm package to install Mozilla Sunbird, and you installed it outside apt/synaptic.
If that means that there doesn't exist a package for your distribution, it is generally not recommended to use packages for other Linux distributions.
So either you update the packages of your current distribution, but you say that it is not possible,
or you uninstall Mozilla Sunbird rpm package and you reinstall it, but with a compressed tar file instead (even compiling it yourself, if binary tar files are not available). So your rpm database will not be affected.
And generally it is preferable to follow the dependencies mentioned in README or INSTALL files.
These are general counsels because I don't use Ubuntu myself.
You may also write to Ubuntu forums to find more specific help for your distribution.
I hope you will find a solution to your problem.
Edit: You may also try the Smart package manager which seems to tolerate broken relations in installed packages. see Smart site