Getting DLL error is one of the most common people used to get while working on the computer. There are different types of DLL error can be found but I too found msvcr120.dll error when I was working on the computer a few days back. Then I found a tool that helped me out, you can also try this and get rid of this error. Here, in the link below, you will find a tool that will help you a lot to fix any kind of DLL error easily:
Its going to be two files that are missing.
msvcr120.dll & also msvcp120.dll.
FF packages them in their builds.
I have seen third party builds that have not.
Now how to tell FF to include them, Id have no idea.
Also, possible you would not run into the situation had you had happened to have the needed Visual C dlls installed systemwide - but not everyone does.