I just upgraded from mozilla 1.6 to 1.7 but unfortunately it looks and feels pretty much like what 1.6 used to feel. I don't mind not having a lot of remarkable features to write home about there was one annoying bug in 1.6 which I hope they have done away with in the new version.
In 1.6 and older versions sometimes when after sending a message the mail client will choke on copying it to the sent folder. It does not matter whether the sent folder is local or on your IMAP store it still chokes. What actually happens is the that 'copying message to sent folder' dialog never goes away. Once I left it running overnight to see what the result would be and in was still running.
There is a variation as well. Occaisionaly the operation would time out and the dialog box will change to say the copy operation failed. Either way the only way to save your copy would be to ask the mail cilent to save the message as a draft and then move it manually to the sent folder. Less than ideal and didn't always work.
Hopefully this bug has been eliminated but can't say for sure because it was pretty hard to reproduce even with the older versions. So let's wait and see.