The account is not related to actual browser use or frequency.

What the email tries to tell you is that you need to go through the login once in a while to keep the account active and its data still available.

If you only have one account, it's most likely the one you used just now to sign in here to post this question — in such case that login flow was enough to confirm it's you and you'll get the next reminder again in a year or two…

I keep getting email messages saying that my Firefox account will be terminated on May 26th because I don't use the browser enough. Is this true? I also use a different browser, but do use Firefox occasionally.

https://support.mozilla.org/en-US/kb/mozilla-account-deletion-inactivity

It is possible to use the Firefox web browsers without using a Mozilla Account or using it very little. They send notices that your Mozilla Account is going to be deleted due to inactivity if it has not been logged into for two+ years. Now that you have logged into support.mozilla.org and posted this thread it should be considered active now.

What a Mozilla Account is used for https://support.mozilla.org/en-US/kb/access-mozilla-services-firefox-account#w_what-services-can-i-use-with-a-mozilla-account

Btw Firefox Accounts was renamed as Mozilla Accounts back on November 1, 2023 to reflect that it is used for other things besides Firefox Sync.