Firefox

Other proposed solutions are valid, I just wanted to add that...

So in my address bar I can type work and it will open up 5 pages that will be associated to that keyword.

To get exactly this behaviour, you can have a bookmarklet with the keyword work and this in the URL field:

javascript:(() => { ['https://ddg.gg', 'https://google.com'].forEach(w => window.open(w)) })();

Some notes:

• You can change the list of URLs with what you want to be open.
• The first time you run it, it will ask you to allow it to open multiple tabs. But it should remember the answer for next executions.
• Is possible to add arguments for advanced use cases