yes, flash is used for youtube

ChaCha said

I have Windows Vista and keep getting notices to update Adobe Flash. I would like to know if it is necessary and also what is it for?

Mozilla blocks versions of Plugins and also Extensions for serious security, stability and malware reasons.

In this case the recent Adobe Flash Player versions was for to address a critical vulnerability. https://helpx.adobe.com/security/products/flash-player/apsb15-14.html https://addons.mozilla.org/en-US/firefox/blocked/

https://www.adobe.com/products/flashplayer/distribution3.html

Should I install it the next time I get a notice?

Yes, but only from the official sources. Don't download and install so called flash from any other site than the official adobe site. If you download and install flash from any other site it might be a virus.

Any time you want or need to check for upgrades, go to the web site of the True Owner of the program in question.

Flash Player Version 18.0.0.194
https://www.adobe.com/products/flashplayer/distribution3.html

See if there are updates for your graphics drivers https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

This is one of the messages I get on the top near the address bar:

"Firefox has prevented the outdated plugin Adobe Flash from running on www.adobe.com"

Don't know what this means.

Flash: Grab the uninstaller from here: Uninstall Flash Player | Windows Uninstall Flash Player | Mac

Then reinstall the latest version.

Flash Player Version 18.0.0.194
https://www.adobe.com/products/flashplayer/distribution3.html

When I click on "Uninstall Flash Player for Windows," a box appeared that said: This plugin is vulnerable and should be updated.

           Activate Adobe Flash
             Check for Updates

Should I just click on "activate adobe flash?" All the other steps seem too complicated. I might mess something up and then I am really stuck!

ChaCha said

All the other steps seem too complicated.

Just follow the step by step instructions.

At https://www.adobe.com/products/flashplayer/distribution3.html, download and install Both Windows ; Internet Explorer and Windows ; Plugin-based browsers