I'm a bit perplexed about the rave reviews. Anyone can get free incoming faxes at https://www.efax.com and outgoing at https://www.gotfreefax.com/ or https://faxzero.com/--and with none of them do you have to put up with unwanted ads (if you use an ad blocker). [I have no connection with any of these.] These have great (free) features: check them out.
UPDATE. After reading the developer's response [below] I did some checking. Regarding incoming faxes, it seems eFax may have changed its model; for me, I can still receive all the faxes I want without charge. (There may be an upper limit, of which I'm not aware. I've never been charged, over many years, and don't believe I provided any credit card information.) Regarding outgoing faxes, faxzero.com currently allows 5 free faxes per day, each up to 3 pages plus cover page; as far as I can see, credit card information is requested only for the premium version. I haven't rechecked gotfreefax.com. Unless you enjoy reading ads, I recommend you check out all three of these alternatives before signing up for this add-on.

I have been using online faxing tools for a while. Recently started using FaxBetter. Their Firefox add-on really works well and makes it free for me to send/receive faxes. I can have my own number at the same time. What else can be better than this?! I love FaxBetter :)

Using FaxBetter for FREE by installing their extension? I mean,
where else can you find that kind of deal? No second thoughts. This is AMAZING!! Thank you FaxBetter!

FaxBetter is so much better than other competitors. When they released the extension so I could get my services for FREE I had to get it! It's truly an amazing deal!