Hi

Thank you for your question about the Firefox Sync service.

This is not what Sync is designed to do. The Firefox Sync service takes a copy of the data you wish to include and transfers it to a second device (typically a mobile device such as a tablet or a telephone) running a copy of Firefox. It can't be used on more than two.

If you have a copy of your Firefox profile for desktop Firefox, you may be able to recover your bookmarks and other data. Please have a read of this article.

If you have a copy of your data on a mobile version of Firefox, please follow these instructions to connect it to Firefox Sync to copy your data.

I have three computers — an older Mac (2010), a newer one (2015), and a laptop Pro (2018). I cannot sync to the older Mac. They are all synced, accounts are signed in, but Firefox will not sync on the older Mac.

You are using the same Sync / Firefox Account on all three, correct?
Which version number of Firefox is the older Mac running now?
And does anything Sync on that Mac?