Also, they could use Firefox on mobile to keep their history and bookmarks in sync across devices in addition to some extensions they find useful, like uBlock Origin as they mentioned. Just because the number of extensions is less than desktop doesn't mean the extensions that do exist aren't game changers for them.
Plus, I'm at two now. Your post reminded me to check how many add-ons I actually have installed, and I realized that HTTPS Everywhere is no longer needed because Firefox itself rolled out an HTTPS-Only setting.
How many do you enjoy?
progress, I guess?
I hate what they've done with Mobile Firefox but it's still better than the alternatives.