Rated 4 out of 5 stars

Some badly configured web servers deliver *.pdf files with "Content-Type: application/octet-stream" instead of the correct "Content-Type: application/pdf", and as a result Firefox wouldn't open a PDF viewer.

This add-on very easily allowed me to workaround that problem.

One feature that I missed: a possibility to get a warning (pop-up box) about a content-type being replaced, such that I remain aware of what is going on.