parkd1 wroteis a nightly build of Fire Fox that I am using.
A nightly build means a build that is performed at the end of each day of development. If you use a continuous integration server, it will generally be configured to build the code and run the unit tests on every check in. At the end of each day you may want to run more extensive tests, regression test and integration tests for example, which take too long to run on each check in and these would be triggered after the nightly build. If you have a full continuously delivery pipeline the nightly build may also be used to deploy the built code to environments for user testing.
For the reasons mentioned above, you should always use a stable version of your browser that has been already well tested and battered with from both functionality and security before deploying it on a production system.
This is not the case for nightly builds, hence the possible loading problem of our website in the Firefox nightly build that you mentioned.
As a reference, here are the release notes for the latest (Stable) version of Firefox 50.0: https://www.mozilla.org/en-US/firefox/50.0/releasenotes/