Comment 1 for bug 147154

Some more details:

After I changed custom Web browser path to Firefox, my firefox asked me if I want to select it as a default browser. I answered "yes" (it was stupid that he asked, it already was a default browser) and I realized that it has changed path to custom one.

Maybe thunderbird does the same? I don't remember if it had asked me in past about being default mail application. If yes, it propably changed setting to custom one and package upgrade don't seeks for that kind of situation, don't seeks for custom paths settings.