I’m not too familiar with Firefox builds. Why are clobber builds common? At first glance it seems weird to add a cache around your build system vs fixing your build system.
I'm the author of BuildCache, and where I work we make thousands of clobber builds every day in our CI. Caching helps tremendously for keeping build times short.
There are a few use cases for local development too. For instance if you switch between git branches you may have to make near full rebuilds (e.g. in C++ if some header file is touched that gets included by many files).
Another advantage as a local dev is that you can tap into the central CI cache and when you pull the latest trunk and build it, chances are that the CI system has already built that version (e.g. as part of a merge gate) so you will get cache hits.