I am aware that Firefox has nightly builds available at https://nightly.mozilla.org/ and was wondering what the difference was between the stable and nightly builds. Are they safe to use or are they primarily for beta testing?
Firefox – the difference between the stable and nightly builds of Mozilla Firefox
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Verify that you are the owner of the Firefox app directory and all files and directories under it (recursively).
If so, enter about:config in the URL bar, search for preference app.update.log, and (if not already set to true) double-click on it to change it to true. Then quit Firefox and restart. After an update check, go to Tools » Web Developer » Error Console, scroll to the end, and look for blue log messages from the update service (AUS). This should show the URL it was using to check for updates and whether there were any errors or updates.
If there are no obvious issues, copy the URL from the log and paste it into the URL bar to manually check the XML to see whether any updates are listed. The URL is generated based on app.update.url (in about:config) and includes the version you are updating from as well as platform, locale, and other information. For example this is the URL generated for Firefox 21.0 beta 6 on Mac OS X 10.8.3: https://aus3.mozilla.org/update/3/Firefox/21.0/20130430204233/Darwin_x86_64-gcc3-u-i386-x86_64/en-US/beta/Darwin%2012.3.0/default/default/update.xml?force=1. Because there is a beta 7 available it should show an update. If there are no updates for this combination of variables you should see <updates> </updates>. If you see no updates but you think there should be updates available, check for components of the URL that do not match the example URL above (other than the specific version and build id) and try replacing them with those from the example. This will determine whether you have an unsupported product, build target, locale, channel name, os version, distribution, or distribution version.
Update 2016-08-06: Firefox now provides an about:profiles page which can be used for creating new profiles. This supersedes Step 1 below, but I haven’t yet tested how it works.
Thanks to @vWil's advice I was able to finally make it work. Here is my step-by-step guide. Note: I'm performing this on Windows 8.1. I'm not sure if the process is different on OS X.
Step 1: Create a second profile
Note: Firefox must be closed while performing this step.
Press Win + R to open Windows’s “Run” dialog, then type "firefox.exe -p" into the input field and press Enter.
This should open a "Choose User Profile" dialog:
Click on the "Create Profile" button and follow the steps. Use a simple name for this profile. I've chosen "nite":
Step 2: Install Firefox Nightly
The download page is here.
Step 3: Set command line flags
Locate the launch icon for Firefox Nightly, right click on it and go to Properties:
In the Properties dialog, add -p nite -no-remote to the Target field:
If you've set a different name for the second profile, use that name instead of nite, e.g. -p foo -no-remote.
That's it
Open Firefox, then open Firefox Nightly using this custom launch icon. Both should be able to run at the same time:
Please let me know via comments if this process didn't work for you.
Best Answer
Nightly builds feature the absolute latest changes, and are considered unstable. They are the most buggy, and will result in a higher crash count than stable Firefox Releases. They are to be used for seeking bugs, and then submitting them to Mozilla developers, so that the Beta and Release channels do not suffer from such bugs.
Nightly is usually many versions ahead - Currently on 9. So you will be seeing features added long before others, but at the risk of crashing more, and potentially broken parts of Firefox. I can confirm this by using x64 Nightly and not being able to use Flash properly anymore (Crashes x64 Flash)
They are to be used at your own risk and discretion. As they say, "These builds are for testing purposes only.", so don't count on Nightly when working.