There is a preference related to drop-down bookmarks menus, and that might work for the drop-downs in the Bookmarks Toolbar. This is how you switch it:

General Warning: Some modifications in the Advanced Configuration Editor can cause unexpected problems. Please only make changes when you feel you have enough information to understand what they do.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste browser.bookmarks.openInTabClosesMenu and pause while the list is filtered

(3) Double-click the browser.bookmarks.openInTabClosesMenu preference to switch the value from true to false -- if you decide to undo this later, you can double-click it again, or click the left-arching Reset arrow at the right end of the row

It's possible you also would need to open bookmarks in a background tab when using that feature:

(4) In the search box in the page, type or paste browser.tabs.loadBookmarksInBackground and pause while the list is filtered

(5) Double-click the browser.tabs.loadBookmarksInBackground preference to switch the value from false to true -- if you decide to undo this later, you can double-click it again, or click the left-arching Reset arrow at the right end of the row

More info on about:config: Configuration Editor for Firefox.

Another way would be to open the Bookmarks Manager window.

Open the Bookmarks Manager; Press the Alt or F10 key to bring up the toolbar, and select Bookmarks. Hot key is <Control> (Mac=<Command>) <Shift> B. Firefox 84+ <Control> (Mac=<Command>) <Shift> O.

From here, you can right-click an entry and select Open New Tab/Window