Firefox Sync is supported in countries around the globe. AFAIK, the only countries where Firefox Sync and USA based servers have been blocked is probably Iran and recently with Russia based users, but that was done by those countries for political purposes, (IMO).

As far as having Sync servers in locations other than the US, I can't see how that would affect the user of Sync services.

Earth is like ~25,000 miles around (circumference) and since electricity travels at the speed of light (~186,282 miles per second) the effect of distance between the the location of a user and a Sync server would be barely noticeable.

Now, if that is important to you, it is possible to setup a local Sync server and Mozilla Accounts server where ever you want. The software is OpenSource and can be used by anyone. https://developer.mozilla.org/en-US/docs/Mozilla/Projects/Firefox_Sync https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Firefox_Accounts/Introduction

If I donate $5 to Mozilla today, would that increase the likelihood I’ll be able to use Canadian and Mexican addresses in Firefox Sync by Firefox 64’s release on December 11, 2018? I very much would find this very useful as I always enjoy exploring just anywhere in the United States, Canada, and Mexico! No sarcasm, no exaggeration, no white lie!

I don't understand what you mean by "use Canadian and Mexican addresses in Firefox Sync." Do you mean a street address, or an email address, or ??

In the country field for saved addresses in Firefox 61.0.1, it’ll only let me have United States. I need Canada as well as Mexico as options.

Oh, I see. I think you mean the "Form Autofill" feature. (Automatically fill in your address on web forms) This is a local feature you can use with or without a Sync account.

It looks as though the next two countries to be added will be Canada and Germany. There are a number of open "bugs" that need to be fixed before that goes live, so considering that there is a 12-18 week development cycle for new features, it could be a while.

Here is a list of the open bugs on the "wiki" page under the country support section:

https://wiki.mozilla.org/Firefox/Features/Form_Autofill#Additional_Countries.28DE.2FCA.29_Support

Please refrain from adding "me too" type comments or requests to the wiki or to bug reports. That can cause them to lock out public comments.