I may be wrong, but I believe this was a feature that was discussed to be added to Firefox policies for. However, I don't think it's been added yet.

There were some old solutions that used to work, but they don't currently.

Are you referring to reading or writing to the root of C:\ ? If so wouldn't it be best to adjust the permissions of "Users" to meet your needs to the root?

We are working towards that for sure but just looking for short term solution to fill one of the gaps.

Thank you for the information. Wish they had a better admx template to apply.

Unfortunately, the WebsiteFilter policy currently cannot allow/block file:// URLs, only http:// and https://

https://github.com/mozilla/policy-templates#websitefilter

There is a bug pending that doesn't have a high enough priority to expect it in the next few versions:

https://bugzilla.mozilla.org/show_bug.cgi?id=1450309

Good to see that it's in Dev and moving forward.