Add more detailed build instructions for Firefox

I use the Firefox version of this extension. The new installation instructions were vague, so I added information to make it easier for me and others to build next time there is an update.
Update Firefox build instructions to `cd build`

So that the xpi is generated in the `build` folder, not the project root. It makes more sense to put the file there, where it will be destroyed with a `make clean`.
Merge pull request #4 from roryokane/patch-1

Add more detailed build instructions for Firefox