Use Downloads.jsm, where available

git-svn-id: https://code.downthemall.net/repos/trunk@3673 43940e1f-b481-bb45-89eb-5c6a4bf3d40a
Fix dta-button initializer

git-svn-id: https://code.downthemall.net/repos/trunk@3674 43940e1f-b481-bb45-89eb-5c6a4bf3d40a
Fix integration messing up context menu (spelling suggestions) (#2473)

Initial patch by v@no. Thanks!

git-svn-id: https://code.downthemall.net/repos/trunk@3675 43940e1f-b481-bb45-89eb-5c6a4bf3d40a