If you are a Firefox Nightly user and you think you have found a bug in Firefox, a regression maybe, then you should wonder if the bug is in Firefox or if your data profile is causing it.
Maybe you changed some setting in about:config? Maybe you have an add-on causing problems? In that case, you might be hesitent to file a bug. Don't be!
Just try to reproduce your bug in a fresh data profile, if you are on Linux or MacOS you can even use this small script that I am using that automates the creation of a temporary user profile, name it Nightly_temp_profile.sh, make the file executable with
chmod +x and launch the script from your terminal. Done.
Here is below the script I am using to do that, as you can see I put it as a gist on GitHub for easy sharing. If you use it, like it or want to improve it, don't hesitate to contact me (pascal AT mozilla DOT com)!
PS1: Yes, this is blatant self-plagiarism of a post I wrote in French in 2010
PS2: This script comes with no guarantee, it has been working for me on Ubuntu for years, it might not work at all for you.