How to disable javascript modal popup ads

The problem is although it appears as if it is a pop up, it is actually just HTML giving the impression it's a pop up. It is often called an overlay. Meaning on some sites, it will be junk (as per the screen shot), but other sites very much required (like a shopping basket preview screen).

Whilst it may block adverts it is possible it could block useful things, such as a discount pop up or cookie policy notifications, which until agreed won't let you proceed.

For FireFox and Chrome, there is a plugin called BehindTheOverlay which may give you what you want (similar to AdBlocker but for overlay). Chrome also has an extra plugin called overlay blocker. It is not perfect for many reasons and as already mentioned, do it at your own risk (as it may stop some sites working as intended).

Lastly, AdBlock plus may be able to help as it can be customised, see this forum for more detail https://adblockplus.org/forum/viewtopic.php?t=18595

You also have the option of disabling JavaScript. I know this is unlikely to suffice, but it's still an option.

You might take a look at the NoScript plugin. It is a bit more convenient than global disable of javascript.