1. Migrate all extensions to restartless/JetPack. Extensions for Firefox are very powerful but may sometimes break when new versions are released, which then the compatibility check feature is introduced to "solve" this. Before the rapid release cycle, I've already seen some users reluctant to update and would wait for extensions to be updated first before updating to new version of Firefox. So now with this cycle, it just annoys them further. If all extensions were done in a more standard (and simpler) extension API first, this would have reduce the impact on the upgrading process. Users don't have to worry so much if the extensions that they depend on, would break.
2. Remove 3rd-party themes and embrace the 'Personas' (simpler themes/skins). Or at least improve theme development into something more standardized like JetPack. Since the Mozilla days, theme development hasn't change much. Extensions have JetPack, but themes have nothing (Personas is entirely a different thing). The case with themes is exactly the same as extensions. I know some users wouldn't upgrade to Firefox because the theme is not updated yet. I'm not sure if anyone still install custom Firefox themes, but themes played a decent role in attracting users (from IE?) in the early days.
3. Implement Electrolysis (https://wiki.mozilla.org/Electrolysis). From my observation, the real problem with Firefox is not the memory usage but its slowness. When users notice that the browser is getting slower (slow startup, slow tab-switching, slow UI response), they probably look at the number of tabs, try to find out the problem and saw that huge GBs of memory used. Firefox is now better at memory management, but the problem is still there. I'm not 100% sure about the implementation details; I'm thinking that if Firefox has Chrome-like multi-process architecture, the interface would be more responsive and memory management would be even better.
So, after this, implement rapid release cycle, enabled by default for new users/installs and prompt existing users to optionally opt in. Make the update process unobtrusive and it would be a win-win situation for everyone.
(I created one of the first 3rd-party themes for Firefox, since the Phoenix days.)