For those of you who are not familiar with Mozilla Prism, it's a tool that allows you to sandbox web applications away from the rest of your day-to-day browsing.  For security freaks (which we should all be to at least some degree), this is an excellent tool, as it prevents data from being shared or potentially stolen between web pages.  It also allows you to clear away the browser navigational elements, making the page more like a real application and giving you more screen real estate to work with.

The big problem with Prism right now is that it's in beta and isn't technically Firefox (even though it's the same exact browser engine).  That means software that sniffs the user agent string specifically for certain browsers has no clue what Prism is, and therefore might not let you do everything you could were you in Firefox itself.  Most people come across this issue with YAHOO Mail.  YAHOO's newest mail client is super-sexy, but in order for it to do all the fancy interactivity that it does, it relies on having a very modern, snappy javascript implementation in the browser.  To make sure the user's experience is top notch, it therefore only lets browsers it knows are good enough into the new client, and forces older or unidentifiable browsers into the old, flat client.  Unfortunatley, it does not yet recognize Prism, and therefore your nice YAHOO Mail sandboxes in Prism won't have access to the newer mail client.

In theory, there's a plugin/template linked to at mozilla's prism wiki that is supposed to fix this problem, but it doesn't seem to work as of Prism 1.0 beta 1.  A user named rotassator on the Mozilla Lab's forums outlined an excellent alternative solution that I'd like to repeat here for searchers' sake.  Here's how to do it:

1. Create your Yahoo Mail sandbox by running Prism and filling in the form, creating a shortcut in your start menu or on your desktop
2. Right-click on the shortcut and go to 'properties', then copy the entire contents of the 'target' field to your clipboard.
3. Goto Start / Run (in XP) and paste this information into the window, then at the end add the text -uri about:config
4. Agree that you're OK voiding your warranty (and don't panic, this isn't a big deal)
5. Right-click somewhere in the window and goto New / String
6. Call your new string general.useragent.override
7. Set it to Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.0.6) Gecko/2009011913 Firefox/3.0.6 - this will make it look like you're using Firefox 3 on Windows Vista
8. Close the window and then run the program from your shortcut normally.  You should now be dumped into the new mail client!

I hope this helps someone!