I think the "Project" feature of ST2 has pretty much the same concept as Firefox's "Tab Groups". The main difference is that you need to save projects to make use of their benefits. That's two files which are generated and updated automatically, you just need to save them somewhere once. Click on "Save Project As..." to create a project from the currently open tabs. I created a new folder where I store all of those silly files. Afterwards you can click on "Switch Projects in Window... (Ctrl+Alt+P)" to switch to a different project. Essentially it's a poor man's tab groups page. Once you have projects there is no problem with using multiple windows anymore. You can close windows without closing their open tabs because the tabs stay open in the project.
I think the "Project" feature of ST2 has pretty much the same concept as Firefox's "Tab Groups". The main difference is that you need to save projects to make use of their benefits. That's two files which are generated and updated automatically, you just need to save them somewhere once. Click on "Save Project As..." to create a project from the currently open tabs. I created a new folder where I store all of those silly files. Afterwards you can click on "Switch Projects in Window... (Ctrl+Alt+P)" to switch to a different project. Essentially it's a poor man's tab groups page. Once you have projects there is no problem with using multiple windows anymore. You can close windows without closing their open tabs because the tabs stay open in the project.