Remove mention to Firefox extension in about.mdx

[giovanniPepi]

Linked firefox extension was removed from the store; update the text to remove its mention

[nickserv]

Do we know why it was removed? Could we publish it again instead?

[giovanniPepi]

It seems that the previous attempts to get the extension on the store running were problematic, and @smeijer is not trying to upload it again so it's in an undefined state currently

testing-library/testing-playground#327

testing-library/testing-playground#344

[smeijer]

Copy from the developer hub:

Approved by Add-ons Review Team about a year ago

This version has been screened and approved for the public. Keep in mind that other reviewers may look into this version in the future and determine that it requires changes or should be taken down.

Thank you!

Rejected by Add-ons Review Team 2 months ago

1. Extensions defining a content security policy that allows eval ('unsafe-eval') are generally not allowed for security and performance reasons. ‘eval’ is only necessary in rare cases.

• devtools/src/manifest.json line 24

In addition the following is required to complete the review:

1. Please let us know to what file is '"sha256-6UcmjVDygSSU8p+3s7E7Kz8EG/ARhPADPRUm9P90HLM=" a reference for and how do you obtain it.

• devtools/src/manifest.json line 24

I'm not sure if @tsuki42 has already attempted to retry, but I'm happy to add others to the Mozilla dev env as well. Just let me know your email address either here, or on twitter dm, and I'll add you.

[tometo-dev]

@smeijer Not yet. Still trying to figure out why they are unhappy about it.

[eps1lon]

We should remove it until the situation is figured out. It has not been resolved in over a month.

[bmulholland]

Closes #1134

[MatanBobi]

@all-contributors please add @giovanniPepi for docs :)

[allcontributors]

@MatanBobi

I've put up a pull request to add @giovanniPepi! 🎉