Good news! You can f inally open the 400 tabs in Firefox like you always wanted! Starting yesterday 64-bit builds of Firefox are available for Windows. A little while back Mozilla had started producing 64-bit builds for Mac OSX as well. 64-bit builds of Firefox for Linux have been available for quite a while now, and if you are running a 64-bit version of any Linux distro in all likelihood you are already running a 64-bit build of Firefox.
There is a lack of good 64-bit browser for Windows, as the only prominent option is Internet Explorer which comes installed with 64-bit versions of Windows.
Right now these builds are only available for the Nightly versions of Firefox 3.7a5 which will eventually become Firefox 4. This does seem to indicate that Firefox will offer a native 64-bit version of Firefox. While these are the official 64-bit builds of Firefox, even before this 64-bit compiled versions of Firefox had been available from third parties.
Currently though, it is more advantageous to use a 32-bit browser, since browsers are quite dependent on the availability of plugins such as Flash and SIlverlight which currently do not have 64-bit builds for Windows. Due to the prominence of 64-bit browsers on Linux though, Adobe does offer a 64-bit version of the Flash Player 10 plugin for Linux. The Java plugin for Windows is also available in a 64-bit version for Windows. This means no YouTube, on the 64-bit Firefox (not even with WebM which is available in the nightlies yet).
Before the release of the 64-bit Flash Player plugin for Linux, there was a workaround called nspluginwrapper, which acted as a wrapper for 32-bit plugins, allowing them to be used in 64-bit versions of Firefox or other browsers. This not only allowed 32-bit plugins to run in 64-bit browsers, but also isolated the browser from the plugin, the same way Chrome, and Firefox “lorentz” isolate plugins from the browser, making browsing more stable by isolating plugin crashes from the browser.
While currently the 64-bit builds of Firefox may not be as advantageous due to the lack of 64-bit plugins and the unoptimized natures of these builds, the move to 64-bit will allow your browser to perform faster with encryption / decryption and other such data intensive operations. The browser will also be able to address more than 3GB of memory, which might not seem that useful now, but might become more important as web applications get richer.
The latest nightly 64-bit builds of Firefox for Windows, Mac OS and Linux are available from: ftp://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk.