Firefox has explicitly used unstable Rust features for years. Their build system opts into this behavior knowing these features are not guaranteed to be stable in future versions. It's 100% unsurprising that using unstable features means your code might break with a future compiler.
The backwards compatibility guarantee of Rust applies only to stable releases. Firefox is weird in that it is enabling nightly features in an unnoficial compiler build that, for some odd reason, they want to tag as "stable". But it isn't.
Technically I believe that they're using the official compiler build, just using an unsupported and undocumented backdoor to enable nightly features in that official stable build.