I have never used Seamonkey; it looks like an all-in-one program like Outlook. Likewise, The Sunbird calendar is one I haven’t tried. I have tried the Thunderbird email program, and have found it wanting. The Firefox browser is a competent program- my main browser, in fact.
I’m looking at Firefox now, but don’t see ‘composer’ listed in it. I have used netscape for years because of ‘composer’. Do you know if Firefox has a similar option?