In 2015, they only spent ~$215M on software development in total, so they couldn't have spent hundreds of millions on FxOS alone.
Plus, I think that view - that FF marketshare was reduced, therefore they weren't investing in it enough - is shortsighted. The improvements that have been landing on FF lately are the result of work being done for a long time. And you can't just dump more money into a project and expect it to be finished sooner - see The Mythical Man-Month.
Well, most of stuff that make recent Firefoxes better, like Electrolysis or WebExtensions, was either initially made for or a fundamental part of Firefox OS.
FirefoxOS produced several useful artifacts, sure, but they were things like device APIs. e10s pre-dates FirefoxOS by many years, and unless there was some very long-term secret plan that they didn't tell the rest of the company about, WebExtensions came after FirefoxOS was shuttered.
B2G extensions were already WebExtension-based when its preview release landed in Firefox 42. e10s was indeed introduced way earlier, but Firefox OS and Fennec were its main users for a while, which for sure has sped up the development significantly. Same with things like APZ.
Not sure how this 2015 figure is relevant as according to Andreas Gal who started the boot to gecko project, the shift back to firefox browser and away from firefox OS happened in 2014.
2011 to 2014, that's 4 years. Enough for firefox OS to gobble hundred millions of dollars.
Plus, I think that view - that FF marketshare was reduced, therefore they weren't investing in it enough - is shortsighted. The improvements that have been landing on FF lately are the result of work being done for a long time. And you can't just dump more money into a project and expect it to be finished sooner - see The Mythical Man-Month.