There is a multitude of Firefox extensions out there, but few are as useful as iCyte. iCyte allows you to save sites (or text from sites) in an archive, instead of just bookmarking a URL. Better yet, it lets you arrange those snippets into projects so that you don't end up with a million-link bookmark menu.
Even if the site or text you archive gets deleted, iCyte saves it--that's crucial for research projects and other messy bookmarking endeavors. A lot of people use note-taking desktop software like Evernote or VoodooPad for work like this, but for most people, those apps are a little too complex and heavy. iCyte lives inside your browser (Firefox or IE), allowing you to add things quickly and view them just as fast. You can also tag and add notes to your pages, keeping everything organized for big projects.
If you're working with other people, you can invite them in on a project so they can see your archived sites and add or edit them.
If you're too attached to your bookmarks to let go, there are other solutions. The best: Mozilla Weave, another Firefox add-on, which syncs not only bookmarks but history, tabs, passwords and other useful junk--so wherever you use Firefox you get all your stuff from home.