I recently enabled the Two column edit conflict beta function, however when I have an edit conflict now I can't select the base version to use as no pop-up window appears for me (so I can't do step 1 "Choose the base version in the popup below."). See this screenshot as an example:
I am using Firefox 54.0.1 on MacOS Sierra 10.12.6. I have AdBlock Plus install, but disabled for wikipedia.org. I have "Open pop-up windows" set to "Block" in my browser, which might be the cause - but this doesn't look like a regular pop-up, and I'm used to seeing an error message appear if Firefox blocks one, or the website complains it can't open one, rather than things silently not working.