There are many reasons for leaks in Firefox, and none of them had to do with not using GC for everything. In fact, there were failed attempts to do exactly that (XPCOMGC), which failed due to performance problems. A lot of those "leaks" were just cases of using too much memory, which pervasive GC actually hurts due to the lack of prompt deallocation (which deferred reference counting loses).

GCs are simply not appropriate for every use case.

Reference counting is not a panacea; once you start wanting to break cycles (which history tells us you will), you start having to deal with stopping the world or concurrent collection. If you don't have thread-safe GC, then you have to either copy all data between threads (which limits the concurrent algorithms you can use) or you lose memory safety.

Finally, your implicit claim (that Rust's safe memory management is more vulnerable to leaks than GC) is untrue. Rust's safe manual memory management is no more vulnerable to leaks than GC. The compiler automatically destroys memory when it is no longer reachable.

`it only took Mozilla 2 decades to get Firefox to not leak memory like a sieve`

I'm sure you're aware that this is quite an unfair/exaggerated statement to make. But yes, I'm all in favor of language features that help prevent memory leaks.

But the reason the smart folks at Mozilla don't just switch to using a GC for all of Firefox (as do none of the other major browser vendors) is due to GC pauses sucking for user interaction. If you don't think that's a concern, or have a solution for it, please elaborate.

I don't see how that's all that unfair. Development on the Mozilla codebase started in early 1998, almost 16 years ago.

Memory leaks waxed and waned as development focus changed. I remember Firefox's memory usage being quite decent around version 2, and even into 3. Later, some things got bloated, though a large part of the memory problem was due to misbehaving plugins. The memshrink project managed to fix even memory leaks across plugin authors' projects.

Also, note that a GC does not automatically mean no memory leaks. For instance, see how leaky Gmail is (was worse according to their dev team).

Well... isn't that the point of Rust? Preventing memory leaks without the need for GC?