Earlier quoted context omitted.
Firefox is an open source project. I'm sure if you pitched in with time and / or money to maintain said features, they'd be much less likely to disappear.
Assuming this is naivete and not a deliberate attempt to mislead... Firefox is an open source project, but here is no openness in the product management. The decisions regarding features to keep or discard are determined by politics internal to Mozilla.
I sincerely hope it's neither. I'm well aware that a third party can't direct Mozilla as to how to channel its resources. They can however ensure there are sufficient contributors external to Mozilla to maintain the features they wish to keep.
Even if the end decision is still made by Mozilla internals, the decision to keep functionality is much easier if they don't have to battle for resources to maintain it thanks to external contributors being willing to do so.
At the end of the day, being an open project does not require the maintainers to do work simply because a subset of users wishes it.