It's more than it sounds like.
As you can see from the breakdown someone posted [0], almost two-thirds of the Firefox codebase is HTML / JavaScript / Python (for tests) / assembly / Java (for Android), none of which is a candidate for being rewritten in Rust to begin with.
If you just look at the portion written in native system languages, Rust is slightly more than 1/3 of that code already and still climbing.
The 10% as pointed out in a sibling comment, while admirable isn't large parts.