Yahoo! recently released a del.icio.us bookmarks extension for Firefox that fully integrates del.icio.us with Firefox bookmarks. As with any Firefox extension, installation is straightforward: just click on the Install Now button on its home page. After restarting Firefox you'll be greeted by a pop-up window that asks for your del.icio.us account login information and whether you want your bookmarks to be imported to del.icio.us. Although I imported the Firefox bookmarks on three different systems without problems, I suggest backing up your profile -- $HOME/.mozilla on Linux -- first, just in case. If you want to switch back to the default Firefox bookmarks, just uninstall the extension.
I've had some synchronization problems with earlier versions of the extension, but the current version 1.3.51 is pretty stable. The only serious problem I've found is that it synchronized only the tags, not the bookmarks, on my desktop system, which had the same Firefox profile I have on my laptop (I had copied the .mozilla directory over). After creating a new profile, this problem was gone.
Del.icio.us bookmarks conflicts with Google Browser Sync and Foxmarks, and will prompt to disable them during the installation. Foxmarks offers similar functionality to del.icio.us but lacks the "socializing" part; it stores your bookmarks and allows you to synchronize them across several different systems and browse them online but you can't share your bookmarks with other people. You can find more information about Foxmarks on this FAQ.
Google Browser Sync is a different story. It's more like a Firefox profile synchronization tool; besides the bookmarks, it stores your browser's settings, saved passwords, cookies, and history on your Google account and allows you to synchronize them across your computers. I use different settings for Firefox on each of my systems and I prefer to memorize my passwords instead of saving them, so this extension doesn't really work for me. I've also grown fond of organizing my bookmarks with tags instead of folders, a feature that neither Foxmarks nor Google Browser Sync have.
While Foxmarks and Google Browser Sync are both nice additions to Firefox, I still prefer del.icio.us over them. The del.icio.us bookmarks extension can improve your Web browsing experience, but it needs some getting used to. If you prefer to keep your Firefox bookmarks and use del.icio.us at the same time, you can try the "classic" del.icio.us extension instead.