How to install Firefox Developer Edition (or Beta) .deb package?
Create a directory to store APT repository keys if it doesn't exist:
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sudo install -d -m 0755 /etc/apt/keyrings
Import the Mozilla APT repository signing key:
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wget -q https://packages.mozilla.org/apt/repo-signing-key.gpg -O- | sudo tee /etc/apt/keyrings/packages.mozilla.org.asc > /dev/null
The fingerprint should be 35BAA0B33E9EB396F59CA838C0BA5CE6DC6315A3
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gpg -n -q --import --import-options import-show /etc/apt/keyrings/packages.mozilla.org.asc | awk '/pub/{getline; gsub(/^ +| +$/,""); print "\n"$0"\n"}'
Next, add the Mozilla APT repository to your sources list:
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echo "deb [signed-by=/etc/apt/keyrings/packages.mozilla.org.asc] https://packages.mozilla.org/apt mozilla main" | sudo tee -a /etc/apt/sources.list.d/mozilla.list > /dev/null
Update your package list and install the Firefox .deb package:
(Replace "devedition" by "beta" for Beta Edition)
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sudo apt-get update && sudo apt-get install firefox-devedition
Source: https://hacks.mozilla.org/2023/11/firefox-developer-edition-and-beta-try-out-mozillas-deb-package/