Not surprised they are losing market share
The thing that once differentiated Firefox was the freedom & control offered to users & developers.
Mozilla seem to have abandomed much of that ethos in favour of restrictions and features that favour the interests of the marketing industry & government.
Undoing all of that on a new installation takes time. Lots of preferences to disable, features to restrict, back channels to knock out.
Meawhile, from an extension dev's perspective, to create an extension for FF now requires dev's to register details with Mozilla and have code approved before it can be distributed. In addition, the browser quietly reports back to Mozilla a list of the add-on installations chosen by users. I resent that, it is intrusive and unnecessary. So I've dropped support for FF.
Given alternatives like SeaMonkey, Iceweasel, PaleMoon, continue to respect the freedom of user's & extension developers' I can't see Mozilla recovering that market niche without a dramatic change of focus.
At present, there is little to differentiate Firefox from the default browser installed by OS makers, and little to motivate most users to replace the default with Firefox.