We're sorry to hear that your Firefox seems to be crashing when you open it. Please perform the following steps to give us a crash report ID that helps us find out more about the cause of the crash.

1. Press the following shortcut to get a Run window: [Windows] + [R]. This should bring up a window that contains a text field.
2. In that text field, enter %APPDATA% and press Enter. An explorer window should open.
3. From that explorer window, double-click on the Mozilla folder, then double-click on Firefox and then on Crash reports. Double-click on submitted.
4. Now, you should see a list of files that contain reports. Go to View > Arrange Icons by > Modified to get the most recent files at the top of the window.
5. Open the most recent 5 files with a text editor and copy the IDs.
6. Paste each ID with bp- into the reply window on the forums.

Thanks in advance!

You can find more information and troubleshooting steps in the Troubleshoot Firefox crashes (closing or quitting unexpectedly) article.

What do you mean "bp-"? I can't find an error report with that anywhere in the appdata.

......

https://support.mozilla.org/en-US/kb/Mozilla%20Crash%20Reporter

If a Crash Report doesn't begin with a bp- that report wasn't submitted to the Mozilla Crash Report server. And if all your reports are like that, you may have the Tell Mozilla about this crash... turned off. That doesn't allow us to help you fix your crashes, as we have no reports to view.

Oh there, I found them. Now how should I post these on the forums and where specifically should I post them?

Alright, update. The reason behind this crash of Nightly is due to an add-on known as Xkit, and it's incompatibility with the latest Firefox Nightly update. Disabling Xkit fixes it, but man I really like xkit.

Then you will have no choice than using an older Firefox version (Aurora, Beta, Release) for the occasion(s) when you really need XKit.