I frequently fall behind on nightly trunk updates, usually because I brush off pending updates and then go for days before restarting Firefox. Occasionally it's because I'm away from my computer on vacation.
Once I'm behind, it's painful to catch up, since the app update service feeds updates to Firefox one day at a time, so I have to Check for Updates, wait for Firefox to find and download the next one, restart the app to apply the update, and then repeat the process over and over again until I'm finally up-to-date.
Bug 306864 is about fixing this on the server side by making the app update service hand nightlies a single update to the latest nightly build, but it doesn't have much traction, so I hacked up a dumb but effective client-side fix in the form of an extension that checks for updates, downloads them, applies them, and then repeats the process until Firefox is fully updated.
Using the extension is simple: just select Make Me Up-To-Date from the Help menu, and the extension will do its thing. Warning: it doesn't prompt you before restarting Firefox to apply an update, and it doesn't tell you what it's doing (except on the console). So make sure you know what you're doing before you use it (i.e. finish filling out and submitting web forms, save tabs you want to access afterwards, start Firefox with a console, etc.).
I've tested it with various Linux nightlies from the last couple months, and it worked as expected except when the app update service or client was busted. Since it uses the same backend components as the regular updater, if Check for Updates doesn't work, then Make Me Up-To-Date won't work either.
If you try it out, post a comment with your results, and include your starting and ending builds plus what platform you're on so I can get a sense of where it is and isn't working. And, of course, back things up in case this horks your build, since it's new, untested code, and it may well be buggy.
Install Make Me Up-To-Date 0.1