I have just upgraded from SmarterMail 15.x to 16.1 and find that reading emails in webmail is very, very, slow in Firefox with it taking around 6 seconds for each email to display in the preview pain.
This use to be almost instant in SmartMail 15, and while 6 seconds to display an email does not sound like long, if you have a large amount of emails to go through then sifting through the several hundred emails I receive overnight is painfully slow.