A few days ago, the logo changed in Firefox Nightly.
I wasn't sure at first if this was a special snippet thing (like Google Doodles), or if they were going to be making the switch permanently to an even cleaner logo. They are doing a whole lot of re-branding lately.
Then, I came across a bug report to change the logo (specifically states Nightly, but could easily apply to release version as well).
Based on the diff, I pieced together my guess at what the regular Firefox logo would look like. Changed a few gradient stop colors and added a blue marble/globe behind it. Here's what I came up with.