Trying to watch Netflix on Debian using Firefox

I had the same problem. Followed the link @eftshift0 suggested in a comment and it worked for me.

This is the part that was most helpful.

#check first what your profile is called and manually save it as
$firefox_profile ls -ld ~/.mozilla/firefox/*.default
mkdir test
cd test
wget https://redirector.gvt1.com/edgedl/widevine-cdm/4.10.1196.0-linux-x64.zip
unzip 4.10.1196.0-linux-x64.zip
rm -rf "${firefox_profile}"/gmp-widevinecdm/*
mkdir "${firefox_profile}"/gmp-widevinecdm/1.4.8.1008/
cp libwidevinecdm.so LICENSE.txt manifest.json "${firefox_profile}"/gmp-widevinecdm/1.4.8.1008/

What I actually did was downloading the zip file with my browser. https://redirector.gvt1.com/edgedl/widevine-cdm/4.10.1196.0-linux-x64.zip

Find the folder where the outdated version of widevine is located. (Ctrl+H) Clicking... Home > .mozilla > firefox > the_folder_that_ends_with.default > gmp-widevinecdm > 1.4.8.1008 (only option for me at this point)

Unpack and overwrite the files with the archive manager to that folder, for me it was exactly the same filenames, different sizes. Replace all.

(Ctrl+H again :P)

I don't remember, but maybe you need to log out of of the profile and back in for it to work.