How to merge 2 windows in Firefox

Solution 1:

Easiest might be just to drag-and-drop tabs from one instance of Firefox onto the other. That gives you control of which tabs to move and in what order they appear.

A multiple tab selection feature was added to Firefox in version 64. If you have a large number of tabs to move and want to do it in bulk, select the first tab, then shift-select the last tab to select the entire range (you can also Ctrl-select individual tabs to create a collection of selected tabs). Then drag the lot to the other window. So moving all tabs from one window to the other can be done with three mouse clicks.

Solution 2:

For Firefox you have an extension:

• Merge-window

and for Chrome I'd been using for years:

• JoinTabs