+++ This bug was initially created as a clone of Bug #1444468 +++ See bug 1426763. Firefox is getting rid of use of XUL overlays. Once that happens we will likely remove overlay support from Gecko. The dependency tree of bug 1426763 has examples of how Firefox is moving away from overlays. At first glance they are mostly getting inlined or replaced with preprocessing. I am given to understand that ETA here is order of "a week" for Firefox to stop using overlays and that we'd then work on the Gecko removal. [bz]
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