FTEXX "Fidelity® Municipal Money Market Fund" is a Money Market Fund(MMF), which means it is a stable value, 1 share of FTEXX will always be $1 USD. It's also part of the "fixed income" group of funds.
I don't actually recommend FTEXX, I just recommend whatever is the highest earning MMF whenever one happens to buy. For the past few months it's been FTEXX, but that will not always be the case.
You can get a list here provided the link still works or from fidelity.com, select 'News and Research' then 'Mutual Funds', then 'By Class/Category' and select 'Money Market', then make sure 'No Transaction Fee' and 'Minimum investment 0 < 2.5k' are both selected and then 'View results'. Then on the results screen sort by YTD (daily) and buy whatever is the first in the list.
At the time of this writing, FTEXX is still winning, and has a YTD earnings of 0.38%, which is not very fabulous.
Note: YTD (daily) doesn't mean you get that earning every day, it means "shows a fund's returns from the first trading day of the year through the most recently ended trading day."