brent Instances is a cool strategy as well
You can have multiple instances of an application like Vivaldi (and others) without the use of workspaces, but together they work quite well. I'm assuming that you know all about workspaces, but just in case you haven't explored them yet, I'll take a chance and respond a little more fully.
The workspaces appear in the bottom panel, and in Budgie you can arrange them via the Budgie settings. Just as other items are placed on the panel, you place the Workspace Switcher there. It takes its place in the center of the bottom panel, and defaults to four workspaces. After returning to the main view, you can add more by clicking the + sign to the right of them. (Budgie limits them to eight.)
You click first on the workspace you intend to use, then start an application in the normal way. It will appear in that workspace only. For more instances of an app like Vivaldi, you right-click on the Vivaldi icon in the task bar, and select New Window from the menu that appears. As I mentioned, I put three instances of it into three workspaces.
From there, it's trivial to click on the instance you want to use at the moment, and Bob's your uncle. By putting them in the same location each time you restart (the workspaces will be empty then), you quickly learn where to click when you want to browse a particular set of contents. I find it extremely useful.
If you use a different DE, as I mostly do, the way you initially set up the workspaces might differ, but the overall procedure is similar. Let me know what I might have left out!
EDIT: For all I know, Firefox may provide multiple instances the same way. I just don't have it installed anywhere so as to check that idea. You might consider right-clicking on its icon to see what its menu makes available.
