[docs] Update firefox version to match requirement of firefox-geckodriver

[oleks]

Description
This fixes the issue that the installation guide can no longer be followed verbatim.

Here is what apt-cache tells me on my machine:

$ sudo apt-cache policy firefox
firefox:
  Installed: 121.0~b7+build1-0ubuntu0.22.04.1
  Candidate: 1:1snap1-0ubuntu2
  Version table:
     1:1snap1-0ubuntu2 500
        500 http://mirrors.digitalocean.com/ubuntu jammy/main amd64 Packages
 *** 121.0~b7+build1-0ubuntu0.22.04.1 500
        500 https://ppa.launchpadcontent.net/mozillateam/firefox-next/ubuntu jammy/main amd64 Packages
        100 /var/lib/dpkg/status

Notes for Reviewers
I discovered this new version by first failing to install the said version of firefox. Then, when trying to first install firefox-geckodriver, I discovered this version.

Signed commits

• Yes, I signed my commits.