Currently the most likely reason is ESET. If you have that installed, try disabling it or adding mozilla. org to its trusted sites. Updating it may also work.

If you don't have ESET, what security software do you have?

Terry Thanks. I tried disabling ESET and adding mozilla.org to the trusted list. Unfortunately neither helped - got the same message (my ESET is up to date.)

You may have to restart your computer. Otherwise, there may be some other cause.

A restart doesn't help. The add-on just won't be added to Firefox.

You should seek help from the their forum for this add-on that was Last updated

   a year ago (Jun 30, 2021)

https://discuss.ipfs.io/c/help/13

OK, will do. Thanks.

Mark your comment as resolved and good luck.