Correct, PaX does not play well with the allocation method that is used by ASAN. Infact, other of the sanitizers are known to cause other problems.
From this gentoo bug report:
PaX hard limits the maximum virtual memory size of a task in security/Kconfig:
...ASAN needs only 2 TB, but it needs them at a very specific address
(to mirror the addresses used by the programm in the lower 2 TB of the address
space), so at least 8 TB address space is needed to fit all the mappings
at their expected addresses.
As such, when ASAN tries to allocate a large range of Virtual Memory for it's Shadow Memory, even while not using the majority of it, it will fail.
The gentoo bug report comes with some proposed fixes, as does this blog post. No amount of extra PaX flags will allow it to work, it has been known to be incompatible for some years. I'd suggest entrusting PaX over ASAN to protect you from exploitation. While ASAN is good for detecting bugs and memory corruption, which is invaluable for developers of large code bases like Firefox, it is not (primarily) designed to prevent exploitation.