Just a short list for me to remember what to download the next time I set up a system for someone else.
First and foremost, the latest release download page for Firefox itself:
Drill down to retrieve the language distribution for your platform of choice.
Note: Adobe has announced that v11 is the last version of Flash that will support Linux — they might as well admit that v11 is the last version of Flash that Linux users will need, because they are already the earliest adopters of HTML5.
The above is the wiki page for the project. Packages for various Linux distributions are available from those distro projects, for example Scientific Linux has icedtea-web in its yum repository.
This plugin lets you download YouTube videos as .mp4 files:
Some developer/sysadmin tools:
Finally, a good PDF viewer implemented as a Firefox plugin for those of us who have lost patience with Adobe: