Mozilla does Not charge for any products or services including desktop Firefox for Windows, Mac OSX, and Linux from www.mozilla.org and Android Firefox on Google Play.

This may be from a random third-party websites that claims to do support on behalf of Mozilla and charges for services and you signed up to it.

Not a "fee". There is no "fee" to download or install Firefox - it's free! Did you make a contribution to Mozilla last month, when the promotion was running on the default home page ? As far as whether that is recurring, I believe that was an option the user could select. One time or monthly.

in case it is a credit card charge for a donation to mozilla, please get in contact with donate@mozilla.org via mail if you have questions about it...