[azakai]

That is not the case.

In fact, Firefox itself did not do this until recent years.

It is true that modern JS development often works that way (as does Rust and some others), but it is not the norm in all ecosystems, and definitely wasn't in browsers.

[sgift]

I very much doubt that Firefox and most other internal software didn't do this until recent years though usually in a more ad-hoc manner, i.e. "oh look, that code does what I want ... but eh .. how can I add that as dependency, far too much work, let's copy it into our tree", which gives you all the downsides of the current state without any of the upsides.

[azakai]

I can tell you as a former Firefox developer that that was not the case. Yes, Firefox has some dependencies copied into the tree, but deciding to do it, and the process afterwards (including updating), were very careful and slow.

Of course there may have been exceptions I am not aware of, and developers are humans that can make mistakes, but that is the overall culture I experienced. It is fundamentally different to the JS/Rust/etc. ecosystem models.