I have Flash and a compatible browser, so is it possible to use the scratch 2 online editor signed in?

I don't think so, but you can save and load the project into the online editor.
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Scratch 2.0 is discontinued and no longer available as part of the Scratch website.

You can still download the offline editor and upload the projects you make to the Scratch website, but logging in to the Scratch 2.0 editor online has been impossible since Scratch 3.0's release in 2019.

As a result, the backpack is no longer accessible in Scratch 2.0. Scratch 2.0's backpack is, in fact, incompatible with Scratch 3.0's backpack; during the transition, all users' backpacks were reset.

If you find any website hosting a Scratch 2.0 editor that claims to allow logging in with your Scratch account, steer clear. Your Scratch account is at risk if you give third-party services your login details.

You can use 2.0 online, but not logged in.
If Flash is set up correctly, when you go to the following url, there should be a fullscreen button with “v392” underneath it:
http://web.archive.org/web/20130618065222/http://scratch.mit.edu:80/projects/editor/

Scratch137 wrote:

If you find any website hosting a Scratch 2.0 editor that claims to allow logging in with your Scratch account, steer clear. Your Scratch account is at risk if you give third-party services your login details.

It could use Scratch Auth though. Also I feel like it would be super rad though if that actually worked LOL

Scratch137 wrote:

You can still download the offline editor and upload the projects you make to the Scratch website, but logging in to the Scratch 2.0 editor online has been impossible since Scratch 3.0's release in 2019.

Also, if it says sth about there not being offline editor downloads, go here and go past the certificate message:
https://scratch.mit.edu/scratchr2/static/sa/Scratch-461.exe (replace exe with “air” or “dmg” for other operating systems and “461” for other versions)