A plugin for the Windows launcher Wox to quickly search for keyboard shortcuts of a number of programs.
In Wox, type:
wpm install Keyboard shortcuts
See more details on the plugin's page.
Type
sc program function
where program is the name of the program, and function is what you're trying to do.
For example, to find the shortcuts for tabs in Firefox, search for:
sc firefox tab
The shortcuts data is collected by DuckDuckGo and is made publicly available via their Instant Answers repo under the Apache 2.0 license.
Check out the full list of those programs.