This is somewhat exaggerated.

The compilation of firefox could take a few hours on some laptop dual-core Skylake CPU from 10 years ago.

Nowadays, on any decent dektop CPU with many cores the compilation of Firefox should take significantly less than an hour, though it remains one of the handful of open-source applications with a really long and non-negligible compilation time.

The Linux kernel is normally compiled much faster than Firefox, except when one would enable the compilation of all existing kernel modules, for all the hardware that could be supported by Linux, even if almost all of that is not present and it would never be present on the target computer system.