Yeah, I did that on 14.01. It wanted to install 15. Then I told it not to update. Next time I ran it something pops up and says "you know you really ought to update... yadda yadda." What a nag!! Jeez! Plus, unlike the old style updater, it's not obvious what version it wants to stick on. It's like they're trying to fool you into going for it. Yeah, there's an update. But we're not gonna' tell you the number because now the numbers don't mean anything anyway. Just stick it on there!
Some improvements are negative.
For a long time I didn't update Thunderbird except for fixes in the same major version. But finally they have some stuff built in that I used AddOns for. Like showing only unread with a button.. no need for AutoCopy etc.. So I update some version way higher. Cool I don't need AutoCopy. But now when I click on another news server, it sits for 3 or 4 seconds before the indicator moves to the other server. It's like it's thinking it over. The old 2.x reacted instantly. Maybe I'll look into putting it back. I really don't need multiple tabs to read my mail.