What Firefly III isn't
Even more important than all the stuff Firefly III can do for you is the list of stuff it can't do for you. If you want a personal finance tool to do this for you, look elsewhere.
Stock and portfolio management
Is not supported by Firefly III. Use something like Portfolio Performance. The net worth you track in Firefly III excludes stock.
Business finances, small business accounting, payroll management
Is not supported by Firefly III. Firefly III is meant for personal finances.
Automated imports from [your bank here]
There are many, many ways to import data into Firefly III. But it won't be automated out of the box, and it won't be as easy as YNAB or Mint.
Advanced accounting and asset management
You can't add your car to Firefly III. The monetary value of your house can't be added to your net worth. If you want to do this, use GNUCash instead.
Prediction, forecasting, future gazing
Firefly III won't support the ability to predict expenses, give you a look into the future or do financial planning. The present is hard enough to manage at the moment.
Automated currency conversion so everything is in Euro's or dollars
It won't be possible to see multi-currency transactions or accounts in a single default currency, by using exchange rates to convert the amounts. The result will always be inaccurate because banks often have steeper rates than the market.
If you submit a transaction in multiple currencies (this is possible) Firefly III won't convert them for you.
Sub-categories, sub-budgets, sub-accounts
Firefly III knows many ways to organise your data, but there are no "sub-levels". Instead, most objects like piggy banks or bills can be divided in "groups" which serve a similar purpose. There are also tags to make things very fine-grained.
Keep track of outgoing loans
Sometimes you borrow your friends money, and you want to keep track of the loan. It's possible to track paybacks in a limited fashion. A full implementation will be included in a future release.
Share your administration
Firefly III is a multi-user environment. But each user is strictly separated from other users.
Changing the monthly cycle to another day of the month.
A month goes from the 1st to the last day of the month. You can't tell Firefly III to go from the 23rd to the 22nd of the next month.