When a user receives an update to Firefox they get either a partial or complete. A complete is nearly identical to its associated installer, and can update any old Firefox version. A partial is a binary diff of a specific old version against a newer one and only compatible with that specific old version. The size difference a partial and a complete can be huge. For example, the complete MAR for 14.0.1, en-US, win32 was 20MB. The partial from 13.0.1 was 7.4MB (even smaller on other platforms, where PGO doesn't make diffing hard).
Until recently, we've only been able to produce partials against a single old version without a lot of extra time consuming and error prone manual work -- meaning that a lot of users who could benefit from a partial weren't receiving them.
With bug 773290 (multiple partial MAR support in release automation) resolved, we can now offer partial updates to many versions without the risk and time cost of doing them by hand. When we shipped 15.0 we offered partial updates to users on 14.0.1, 13.0.1, and 12.0 - which collectively represented just over 75% of our installed userbase. For future releases it's possible we'll offer partials to even more previous versions.
I'm sure some of you are asking why we can't just do partial updates for ALL old releases. There's a couple of reasons for that. Most importantly, there's big diminishing returns on partial updates. The 13.0.1 -> 15.0 partial was 12MB, the 12.0 -> 15.0 partial was 14MB, and the complete was 20MB. The further you go back the less you gain from getting a partial. Secondly, computing partial updates is not computationally cheap. We ship 88 locales across 4 platforms -- this works out to about 350 partials that need to be calculated. This can be made cheaper through caching and parallelization, but it still ends up adding about 45min to the running time of the release automation for every extra version we want partials for. In turn, this delays QA and other things in the critical path of shipping.
I also want to point out that this work does NOT apply to Nightly or Aurora. We have no plans to offer multiple partial updates on those channels at this time. Due to their relatively low userbase and very high frequency of change (almost every 24h), the cost/benefit just doesn't work. However, we will be looking at using this on Beta where the userbase is much larger and the rate of change slower (about once a week).
A huge thanks goes out to Rail Aliev and Nick Thomas, who helped work out the design, wrote some parts of it, and provided reviews. We couldn't have had this ready for 15.0 without their help.