Firefox for Android also lets you opt in to help improve Mozilla Location Services whenever Firefox makes a location request. This is equivalent to using the Stumbler app (and you should check its battery/RAM usage to make sure this setting is acceptable for you):
It may just be a matter of data disparity - who really has more info to correlate multiple data points and triangulate you? Big G or one offline data provider?
That said, I've actually (anecdotally, I guess) seen an improvement in the speed with which my device can locate me with LineageOS + microG. I've been using Unified NLP with the GSM location backend that you can find on F-Droid.
Not sure how well this would work out if I, say, road tripped through an area without cell service - not sure if it uses ANY carrier's tower it can find (that your baseband supports) or just your carrier's towers. Waze finds me much faster on launch, though.
Does anyone have any idea why this is? Are the open source location provider backends just not as robust as Google's?