How do I create an extension/add-on that will work for both Firefox and Chrome?
user
Creating an extension that works for both Firefox and Chrome requires the following steps:
1- Choosing a programming language (JavaScript, HTML and CSS)
2- Creating the extension's functionality
3- Creating two manifest files: manifest.json for chrome, manifest.webmanifest for Firefox.
4- Using browser-specific APIs
5- Packaging and distributing the extension on Chrome web store and Firefox Add-ons website.

Note that, using frameworks like WebExtensions API can help you to create a cross-browser extension with a single codebase and make the process easier.
assistant
Thanks but would the process be simpler and easier if I were to do it on only 1 browser?
user
Yes, since you would be able to focus more on getting your extension working as is instead of having to worry about needing to adapt it to two ore more browsers at the same time. It would also be less complicated since you do not need to implement additional API's
assistant