3.5.6 seems more stable, but with the previous version, I was experienced a crash a day with lots of tabs open, followed by a message 'woops, this is embarrassing, Firefox seems to be having trouble opening your tabs....', but then it would open them.
Thunderbird has also recently been playing up, refusing to manually get mail with a message 'please wait while the folder is being processed'. The only advice I found for this was for Macs, not Windows. Eventually seems to clear. I found with Outlook Express and Outlook, very large mail records messed up those programs, so I have been deleting swathes of historic emails to bring the mail file down to a more reasonable size.