I guess I can try it and see if it will work on Firefox. Other than that I have Windows 7 PC. I’ve never heard of an app on anything but a cellphone.
I think it’s specifically a Chrome browser extension, but maybe they have a version for Firefox too.
App is short for application software. Apps are usually associated with smart phones. Most apps will not run on a flip phone as it is too simple of a platform. Note that both flip phones and smart phones are “cell” phones based on cellular technology.
There are lots of applications: Quicken is one, as are Word, Excel, etc. Apps originally were written for PC type computers running Windows and other operating systems but are now written for smart phones as well.