The roads I take...

This Tuesday, I landed "omnijar" being used for SeaMonkey, which greatly reduces the number of files installed on users' machines by putting a large amount of all the UI and other things the application needs into a single zipped-up file called "omni.jar" (Firefox and Thunderbird also have switched to that recently). As most of those contents need to be read on every launch of the application, and opening files is costly in the operating systems, this step should improve startup time of SeaMonkey starting with today's nightlies and the upcoming SeaMonkey 2.1 Beta 2.
We're also shipping all built-in extensions as XPI files inside the extensions folder of the application now, which more or less is the equivalent to the omni.jar method but for add-ons.

If you're building your own versions of SeaMonkey, note that this is switched on by default and you need to clobber your objdir to make builds work correctly after this switch. You should be able to turn this off with the --enable-chrome-format=jar (or any other chrome format you used before) option, which might make things easier for developing but gives you a different and probably slower build than what we ship to testers and users from now on.

Please make us aware of any problem you're seeing, e.g. with updates, which we tried to make work fine, but could not test before landing this switch.