Steam extension for Firefox?

Augmented Steam does it along with a host of other useful features: https://addons.mozilla.org/en-US/firefox/addon/augmented-steam

Don't know about the bundle feature though although, you could use this site (I know, not an extension but still): https://isthereanydeal.com/ . Just type the name of the game in there, click on it's link and scroll down to the "bundles" section.

Bundled: Augmented Steam, also current and historical low on all official resellers (including Steam)
Historical low on Steam only: SteamDB extension