[lxt]

The stub itself pulls down the full installer, and you can always get that from download/FTP if needed.

The intention is to provide a better download experience: there was a gap between the number of people who begin downloading Firefox (counted via hits on download.m.o), and the ones who open it for the first time (counted via hits on the firstrun page). The stub installer is intended to address this. It also checksums the full file to make sure you're getting what you think you're getting.

(Disclaimers: I work for Mozilla, and am responsible for download.mozilla.org among other behind the scenes web things. I don't work on the installers, but we had to change the download scripts to support the stub. I do not speak for Mozilla.)