With GitLab specifically as an alternative: GitLab made it very clear a few years ago that they weren't particularly interested in hosting large-scale free projects when they introduced the Open Source Program as the only path to using GitLab for FOSS. I've heard over and over again that this process is painful and not worth the effort, and it has a bunch of extra requirements that would likely be dealbreakers for Mozilla [0]:
* "the Open Source Project does not, and does not seek to, generate profit from the sale or licensing of the Open Source Software to which the Open Source Project relates, or the sale of any services related to such Open Source Software;"
* "The Open Source Project agrees not to (nor to authorize any third party to): ... (b) modify or create any derivative works of the GitLab Software ... (d) copy ... the GitLab Software"
That last part is especially problematic for everyone: in order to use GitLab.com for a FOSS project you have to renounce your right to modify (or authorize others to modify) or to copy the FOSS version of GitLab. This might have just been lawyers adding boilerplate without thinking it through, but that in itself is evidence of a major problem at GitLab.
So, GitLab is out. Aside from GitLab Mozilla could have chosen maybe Codeberg, but with the entire point being to remove barriers to new contributors it makes sense to go with the option that almost all such possible contributors are already on.
* "the Open Source Project does not, and does not seek to, generate profit from the sale or licensing of the Open Source Software to which the Open Source Project relates, or the sale of any services related to such Open Source Software;"
* "The Open Source Project agrees not to (nor to authorize any third party to): ... (b) modify or create any derivative works of the GitLab Software ... (d) copy ... the GitLab Software"
That last part is especially problematic for everyone: in order to use GitLab.com for a FOSS project you have to renounce your right to modify (or authorize others to modify) or to copy the FOSS version of GitLab. This might have just been lawyers adding boilerplate without thinking it through, but that in itself is evidence of a major problem at GitLab.
So, GitLab is out. Aside from GitLab Mozilla could have chosen maybe Codeberg, but with the entire point being to remove barriers to new contributors it makes sense to go with the option that almost all such possible contributors are already on.
[0] https://handbook.gitlab.com/handbook/legal/opensource-agreem...