Yes, this has been a long-running problem, and was the impetus for Mozilla's attempt at FirefoxOS/boot-to-Gecko.
The current mobile situation is:
* iPhone: can't run Firefox there at all, really. There's Firefox for iOS based on WebKit like everything else on iOS, but I don't even know that StatCounter would count it as "Firefox".
* Android: Google for a long time had (and in many cases continues to have, as far as I know) agreements with OEMs that forbid a default browser other than Chrome. In some cases those agreements forbid preinstalling a non-Chrome browser at all, even as non-default. So you have to rely on people downloading an extra browser from the app store, and people don't do that much.
In addition to all that, "mobile" sites tended to be written to "WebKit", not "standards", for a while for various historical reasons. Unfortunately, at this point that's pretty deeply ingrained and people continue to do that, albeit with "Blink or WebKit" replacing "WebKit", which entrenches the problem.
I don't understand why they sunk so much money into Firefox OS when they could have much more easily released an Android spin that had Firefox by default, along with other privacy enhancements.
I think they missed a huge opportunity in about 2014/2015 to do what CyanogenMod was doing, as a clean/no-nonsense power user variant of Android. Ultimately the Cyanogen company leadership made poor choices and killed their relationship with the largest phone manufacturer that was shipping their OS on the phones (OnePlus), who decided to go and re-implement the same features in their own oxygenOS android build. Thereby killing Cyanogen as a company.
My understanding, and I could be totally wrong on this, is that at the time such an Android spin would not have been able to use things like Google's app store, and possibly things like Maps and whatnot, due to not having Google's browser as default.
The current mobile situation is:
* iPhone: can't run Firefox there at all, really. There's Firefox for iOS based on WebKit like everything else on iOS, but I don't even know that StatCounter would count it as "Firefox".
* Android: Google for a long time had (and in many cases continues to have, as far as I know) agreements with OEMs that forbid a default browser other than Chrome. In some cases those agreements forbid preinstalling a non-Chrome browser at all, even as non-default. So you have to rely on people downloading an extra browser from the app store, and people don't do that much.
In addition to all that, "mobile" sites tended to be written to "WebKit", not "standards", for a while for various historical reasons. Unfortunately, at this point that's pretty deeply ingrained and people continue to do that, albeit with "Blink or WebKit" replacing "WebKit", which entrenches the problem.