Running online updates today (opensuse 12.2), gave me firefox 18 and similar updates to thunderbird.
I am mostly not noticing any difference, except for the way that cookies are stored. They are still saved in “cookies.sqlite” in the profile directory. But the format of entries in that sqlite database has changed.
I notice this, because I have a script that I occasionally run to extract cookies to classic netscape format (a “cookies.txt” file). And when I tested that script, it had stopped working. Fortunately, I don’t seriously depend on that script. I’ll probably rewrite it in a few days, but I don’t see it as an urgent problem.