For Mozilla, 2015 has been a year of large challenges, with a shift in funding sources and increasing competitive pressures across the desktop and mobile markets. The biggest challenges for Mozilla, however, are likely yet to come in 2016.

One of the biggest challenges for Mozilla in 2015 and into 2016 is funding; 2015 is the first year in a decade that Mozilla didn't rely on Google for the vast majority of its revenue. Mozilla signed a five-year partnership deal with Yahoo at the end of 2014 that went into effect in 2015.

Mozilla has not yet provided full financial transparency on its 2015 earnings but did report on its 2014 financials at the end of November. Mozilla chalked up consolidated revenue of $329.5 million in 2014, up from $314 million in 2013. That's a massive gain from the 10-year period in which Mozilla's Google deal has been in place, with 2005 revenue of $52.9 million.

"Mozilla receives royalty income from contracts with various search engine and information providers," Mozilla's audited financial statements explain. "Revenue from these contracts is determined by the search and information providers based upon end-user activity or as contractually agreed to."

The Google deal expired in November 2014, and as such, the full impact of the shift to Yahoo won't be publicly known until Mozilla reports its fiscal 2015 financial results in 2016.

A decade ago, when Mozilla first engaged with Google, there were only desktop browser clients. In 2015, that's no longer the case; there are now large volumes of users for mobile browsers. It's a space where Mozilla is actively competing with a number of expanded efforts.

For Android users, Mozilla continued to update and iterate its Firefox for Android browser in 2015, providing users with an alternative to Google's Chrome browser, which is typically the default.

On Apple's iOS, an operating system on which Mozilla has been absent for years, 2015 marked the return of Firefox on Nov. 12. Unlike Firefox for Android, which uses Mozilla's long-established Gecko rendering engine, the iOS version of Firefox uses WebKit. Mozilla's use of WebKit marks a departure from the open-source organization's long-standing policy of only using its own rendering engine. It also marks an acceptance that Firefox needs to grow across all platforms to continue to be successful.

One area in which Mozilla did not overtly succeed in 2015 is with FirefoxOS. With both iOS and Android, the Web is not a first-class citizen; instead, both mobile operating systems favor the model where the app dominates. FirefoxOS was an attempt to change that, restoring the Web to the first-class position it holds on the desktop.

The name FirefoxOS came into existence in July 2012 after Mozilla rebranded its Boot2Gecko project. The effort was initially led by Andreas Gal, who was named CTO of Mozilla in April 2014 but left the organization in June 2015.

Back in March, in an interview with eWEEK, Gal highlighted the growth of FirefoxOS, which at the time was powering 17 different smartphones across 40 different markets around the world. Ten months later in December, Mozilla decided to exit the smartphone market. Firefox OS isn't dead, though, not by a long shot. As an open-source effort, there is still the potential for other interested parties to push the system for smartphones. Firefox OS is also still a player in the embedded market, with Panasonic currently offering a Firefox OS TV.

While Firefox is Mozilla's best-known effort today, Mozilla still helps support the Thunderbird email project. It's important to remember that the genesis of Mozilla is as the open-source engine that powered the Netscape browser suite, which included an integrated browser and email client.

Mozilla has been trying to figure out what to do with Thunderbird for years, and 2015 was no exception, and 2016 isn't likely to be either. Simply put, Mozilla's efforts and users are on the Web, which is why the Firefox focus matters.