Not sure if many people know this but Adobe Flash player will be permanently removed from Google Chrome December 2020.
Im not too sure if this will follow along onto Firefox, but It'll make a huge impact on browser games ect, that use Flash Player.
Adobe Flash Player to be discontinued.

How will we be able to play TFM once flash player is removed?
Will we have to download it? I know theres a download version of TFM but I haven't used it in years.

Hello!

As you already stated, Transformice has a standalone version that you can download. You can find it on the front page of www.transformice.com.
It is also possible to play Transformice through the Steam standalone. So no, Transformice will not close. It is however likely that the game will become a standalone-only game.

You can discuss further how browsers will stop supporting Flash in 2020 here!

In the future, please use the search function before posting to avoid recreating an existing thread.
Any other questions regarding the matter can be asked in the thread linked above or in the Questions & Answers thread.

Thank you!