Rated 2 out of 5 stars
UPDATE: Same issue continues with v2.8.2.
V2.8.1 kept freezing in FF9, so I went back to 2.7.9 and adjusted the install.rdf to raise its compatibility level (I set it for FF10.) When this began to have increasingly frequent (but still intermittent) problems I tried v2.8.1 again, but it's the same issue: Simple Mail periodically fails to connect to the mail server. After a day or sometimes just an hour it works again. This is not a server issue, since Thunderbird works fine. Simple Mail was once the gold standard of reliability (although it lacked some features). Now it has features but is less and less reliable.
>> Update: Changed v2.7.9 Install.rdf to allow maxVersion=12.* and the revised v2.7.9 works with FireFox 11.