In light of the fact that we've switched to clang-cl for our Windows builds[1], 
we are planning to drop support for compiling Firefox with MSVC in the near 
future[2]. Our estimate is that this will happen sometime in Q1. Supporting 
more than one compiler is a maintenance burden and we've already seen 
developers spend considerable time getting their patches that work with 
clang-cl to build with MSVC. We are currently blocked by the fact that our 
aarch64-windows builds are still using MSVC and we are waiting on upstream 
clang-cl work to switch those builds to clang-cl. Once that takes place we no 
longer have a compelling reason to continue supporting MSVC.

To preempt the question--when this happens we intend to make MSVC error in 
configure, and not just move MSVC to Tier 3 "patches welcome" status. Our 
reasoning is that Tier 3 configurations still create work: developers spend 
time building in those configurations, and lack of CI coverage means that when 
they inevitably break they waste time trying to fix things. Bugs get filed, and 
other developers waste time trying to help or reviewing patches to fix things. 
Explicitly unsupporting MSVC is the best way for us to convey the fact that 
developers should not be using it and we will not accept patches to fix issues 
that only affect MSVC.

If you have specific reasons for continuing to use MSVC please let us know. If 
there are deficiencies in clang-cl compared to MSVC we should get them filed 
and fixed.


1. https://bugzilla.mozilla.org/show_bug.cgi?id=1443590
2. https://bugzilla.mozilla.org/show_bug.cgi?id=1512504
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