I've been using Firefox for a while now and I am quite satisfied by it. I have always heard that it is open source, and I recently tried looking on GitHub to see if there was some sort of repo with the source in it, however, I wasn't able to find such a thing. Is the Firefox source code available to view?
Yes, Firefox is 100% open source. Its source code is available at https://hg.mozilla.org/mozilla-central
Adding to Mureinik's answer, here is few additional information on open source projects.
An important part for any open source project is its license.
Making a source code available in the public domain doesn't make a software open source.
The author(s) has to give the permissions to the user to freely use it, modify it, and redistribute it.
This permission can be given by adding any Open Source License to the source code. It is generally a file with the name LICENSE.
Firefox is licensed under Mozilla Public License open source license. Thus it allows us to freely use it, modify it, and redistribute it, hence it 100% open source.