docs: Include MV2 build commands in README (#26486)

The build instructions in the README are misleading at the moment because they make no reference to MV2. This could lead to contributors using MV3 builds with Firefox, leading to errors.

They have been updated to recommend the MV2 build commands when working with Firefox.

README.md

@@ -57,9 +57,9 @@ If you are not a MetaMask Internal Developer, or are otherwise developing on a f
   - If debugging unhandled exceptions, you'll need to add a value for `SENTRY_DSN` [Sentry Dsn](https://docs.sentry.io/product/sentry-basics/dsn-explainer/), see [Developing on MetaMask - Sentry](./development/README.md#sentry).
   - Optionally, replace the `PASSWORD` value with your development wallet password to avoid entering it each time you open the app.
 - Run `yarn install` to install the dependencies.
-- Build the project to the `./dist/` folder with `yarn dist`.
+- Build the project to the `./dist/` folder with `yarn dist` (for Chromium-based browsers) or `yarn dist:mv2` (for Firefox)
 
-  - Optionally, you may run `yarn start` to run dev mode.
+  - Optionally, to create a development build you can instead run `yarn start` (for Chromium-based browsers) or `yarn start:mv2` (for Firefox)
   - Uncompressed builds can be found in `/dist`, compressed builds can be found in `/builds` once they're built.
   - See the [build system readme](./development/build/README.md) for build system usage information.