pfSense

Use a touch screen so you don't have to scroll to the bottom for access to the horizontal scroll bar. Just swipe left and right.

You might check out the following. It replaces the displayed underscores in aliases with a space so they can line wrap (only in display, not in config).
https://forum.pfsense.org/index.php?topic=106803.msg594947#msg594947

Also might consider adapting that to replace colons with colon plus zero width space so that the IPv6 addresses can line wrap too. Though that may cause problems for cut and paste in to an address field of some tools.

Another possibility would be to put those long addresses into a much shorter alias. Plus an alias can be much more human friendly and recognizable.

Well lets assume quite some people will still use a mouse to configure their firewall through its webgui..

The issue is not just with long IPv6 ip's. Its also with states+bytes+serveralias+portalias+serveralias+portalias+queue+gateway+schedule+descriptions things that can make it 'just not fit'. And they dont need to all be on 1 rule, having a few of these items spread over a hundred different rules still makes the whole table to wide. In the screenshot only 3/4rd of the actual with can be seen, but even when only the last 50 pixels(actions column..) cant be seen its already inconvenient..

As for selecting a ip adress from the list which your mention, that doesn't work either as doubleclicking edits the rule, and dragging moves the rule. There is no easy way to select some of its text. Perhaps worth another PR.?