Greetings scott212
If you buy a new tablet, in any case, Firefox will work on it regularly, unless of course your new tablet will not older versions of Android. Developers will be announced in advance of the termination of the support, just follow the news ;)
PS. Also, I advise to look this article.
Thank you for contacting us, I hope my answer helped you!

Firefox for Android's minimum version of Android is 4.0. Your 4.4 tablet will run Firefox for Android.

Dropping support generally requires that the Android user base for that version is a few percent or lower. Currently according to Google's stats Android 4.4 (KitKat) is 27% of active devices. https://developer.android.com/about/dashboards/index.html