In Kubuntu 11.10, my default mail and browser applications are Thunderbird and Firefox respectively.
After installing Google Chrome (primarily for its integrated PDF viewing capabilities) I found that Thunderbird was suddenly opening http/https links in Google Chrome rather than Firefox.
After double checking that Google Chrome was not my default browser and that Firefox was my default browser, I still experienced the same behaviour.
After a search, I found this link that explained what happened in a Fedora setup:
I did what was advised there:
1. sudo edit /usr/share/applications/defaults.list
Note that in Kubuntu it is firefox rather than mozilla-firefox that is the replacement string.
I do not know how this default setting happened in Kubuntu as gvfs-open is not installed. Anyway, the default was set in a sneaky fashion on my system without my explicit intervention, and I am grateful that David at GeekLab found out what had happened and how to fix it.
I hope this helps others.