Mozilla Weave Sync
Weave Sync, a prototype of a new Mozilla service, can synchronize your bookmarks, passwords and other data across multiple Firefox installs.
After installing the Weave Sync add-on, head to Tools | Weave | Sign In to set up a new sync account. You'll pick a username and password, and also an encryption passphrase that will be used to encrypt your browser data before it leaves your PC.
You can synchronize your choice of bookmarks, passwords, browsing history and open tabs across browsers installed on multiple PCs. For example, if you set up Firefox at home and Firefox at work on the same Weave Sync account, you can add a bookmark to one browser and have it automatically appear on the other.
Such synchronization is infinitely more convenient than the archaic practice of e-mailing yourself links, but keep in mind that Weave Sync is a work in progress. Mozilla lists the service as a prototype that's "Ready for testing," and it lacks some polished found in similar, finished services, such as the ability to fine-tune initial setup to replace existing bookmarks with those stored on the server.
If you want to give Mozilla a hand testing what may become a very nice service, give Weave Sync a try. If you'd rather use an already-finished add-on, try Xmarks (formerly Foxmarks), which offers nice features for sync'ing bookmarks and passwords, but doesn't handle browsing history or tabs.
Note: This download link takes you to the Mozilla.org add-on site, where you can automatically install the file into your Firefox browser.