https://hg.mozilla.org/mozilla-central/rev/c224fcf24bfa

This breaks builds where the milestone doesn't end in "a1":

 0:03.91   File "/media/external/dev/gecko-dev/python/mozbuild/mozbuild/configure/__init__.py", line 1289, in wrapped
 0:03.91     return new_func(*args, **kwargs)
 0:03.91   File "/media/external/dev/gecko-dev/js/moz.configure", line 1237, in <lambda>
 0:03.91     depends(js_version)(lambda x: x.version[-2] if x.version[-2] in "ab" else None),
 0:03.91 TypeError: 'in <string>' requires string as left operand, not int
Error running mach:

This seems to fix it (as in, I can build):

diff --git a/js/moz.configure b/js/moz.configure
index 6bc1e6cf797e..f37eccff7441 100644
--- a/js/moz.configure
+++ b/js/moz.configure
@@ -1234,5 +1234,5 @@ set_define("MOZJS_MINOR_VERSION", js_version.minor)
 set_config("MOZJS_PATCH_VERSION", depends(js_version.patch)(lambda p: str(p)))
 set_config(
     "MOZJS_ALPHA",
-    depends(js_version)(lambda x: x.version[-2] if x.version[-2] in "ab" else None),
+    depends(js_version)(lambda x: x.version[-2] if str(x.version[-2]) in "ab" else None),
 )

Please file a followup bug and send your patch to phabricator.

(In reply to Mike Hommey [:glandium] from comment #6)

Please file a followup bug and send your patch to phabricator.

I opened bug 1746550. I'll try to submit a patch tomorrow, but if I don't have time I'll be afk until 2022.