I personally believe that Thunderbird has declined in usability with the past few renditions called updates. I like it less today than I did a year ago.
One of my big complaints, wrapped lines in outgoing mail — I don’t want them wrapped, often url links are chopped off, and it’s just not pleasant.
I found a plug-in that worked on a case by case basis — about every third email I send I have to “unwrap”. Not so happy — until tonight!
Here is my salvation:
Tools > Options > Advanced (tab) > Confi Edit (button) > Confirm on Break Warranty Screen > Filter for “wrap” and change the following two settings by right clicking on them, (a) mail.compose_wrap_to_window_width sett to true, and (b) mail.wrap_long_lines set to false.
At first I also had mailnews_wraplength set to 1022, but in the end I think that is unnecessary as that is for news — I think.
Here is a screenshot of how to disable word wrapping in Thunderbird 3.0.x and above.