I’d say hold off for a second there (or a few hours, or maybe a day or two). I’m now doing more proper measurements and trying out different linking flags too. Stuff is noisy and what worked a few times, might still not be reliable/good enough for the long term given the infra.
I did switch to the NUR package out of curiosity, which also works fine on my machine (unlike mid-December). It took a few hours to build and seems to be an older version = “140.7.0esr-bb17” than the binary release I’ve found and updated my custom betterbird.nix to a few days ago = “140.7.1esr-bb18” - though I’m not sure what that “bb*” part means in the version string.
Alright, at this point the librewolf flags PR has landed and it does seem like it has gotten better. I also added a few metrics to the issue. So possibly worth a try, especially switching from full-lto to thin-lto and dropping the symbols made resource consumption noticeably smoother during compile.