Could Merge be a feature of one of your extensions, or of that website you're using? It doesn't sound like a built-in feature.

You also can call up Responsive Design Mode using the menu. Either:

• "3-bar" menu button > Developer > Responsive Design Mode
• (classic menu bar) Tools > Web Developer > Responsive Design Mode

Was perfectly fine up until FF48. After upgrade, it pull up the "merge" dialog bar.

Still works normally for me on this site...

Could you test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

OK in safe mode it goes back to normal. So clearly with the upgrade to 48 there is a conflict with a plugin. THX

Check the extensions; Safe Mode doesn't disable plugins.

You can view, disable, and frequently configure extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then cast a critical eye over the list on the right side.