Actually there have been historical problems doing an 'update' just from one version to the next. I have found that the only way to do Mozilla products right is to use the full-install version. I uninstall the old too first and then install the new one. I've been using Mozilla products for many years and have never had any problems updating by doing it this way.
I have been fortunate in that I use the internal update and just download the patches and have had no problems with Firefox. I just started using Thunderbird and this update went well using the internal update.
I just use the automatic update aswell, no problems here,
Though I haven't compared the stability to a fresh install.
When I see 'missing' versions I just assume they where internal (without a public release)...
Updates will work just fine 19 out of 20 times. But there have been many cases where every update brings problems for users on different Mozilla forums (just look at Mozillazine after any given update). Things like "Firefox won't shut down" ... "my bookmarks are gone" ..... lost my profile" .... "running very slowly" ... "won't start". Things like that. In almost EVERY case, the user has done an update and not a full install. Not that I'm saying that you always need to do it, but I would suggest doing it at least every third version to 're-stabilize' the program.
But even though most of these are problems with Firefox updates, I don't take any chances with Thunderbird because they share a lot of similar 'parts'.
It might be a small inconvenience to download and add the full install, but I find that it's a small price to avoid a potential problem.