In general I'd advise against using the MailNews portion of SeaMonkey if you're also using the profile for web browsing. (MailNews is the guts of Thunderbird.)
MailNews results in a non-trivial amount of main-thread I/O that is going to badly jank your browser experience. The good news is that the worst of it will be when using the MailNews-related UI like switching folders for display, but it's still going to have an impact.