If you're referring to our XPFE builds, they only supported 1.9.0a5, so
you'd probably be able to compile it against Gecko 1.9.0.x, but of
course it wouldn't have the extension manager, form autocomplete, feed
preview, vista-compatible shell integration, fast location bar
Perhaps someone will take an old pre-alpha 1 build of SeaMonkey 2.0 which
supported early Gecko 1.9.1 before any of these UI changes were added, drop in
the current version of Gecko 1.9.1 and release that as a user-supported custom
build, a sort of SeaMonkey 1.5.
Warning: May contain traces of nuts.
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