In short, on some sites in certain circumstances (details below), LibreJS enters an infinite loop which blocks the browser from loading anything whatsoever on any page, including newly opened pages, until the browser is killed. While I would expect LibreJS's script blocking to break some sites, I don't expect it's script blocking to ever block all loading of all parts of all webpages.
If LibreJS is disabled, then these issues immediately vanish. Additionally, I tested blocking scripts with the NoScript plugin instead and I could not cause any such issue no matter what combination of scripts I tried blocking or unblocking. For this reason, I believe the core issue is something in LibreJS as opposed to bad configurations with the sites themselves. The simplest way I've found to reproduce the issue is to do a clean install of GNU IceCat 60.7.0 and visit https://lichess.org/ to promptly trigger an infinite loop that freezes the browser. I've reproduced the issue with LibreJS 7.20.2 and seemingly any version of IceCat since at least version 60, though earlier versions may have the issue as well. I've also verified that the issue is present when LibreJS is installed as a plugin to the latest version of Firefox (version 76.0). I suggest testing with Lichess's website since so far, it's the only site I've found that reproduces the issue with 100% consistency. Additionally, Lichess is known to be open source under the GNU AGPL 3.0, so in theory there should be no risk of running non-free JavaScript while testing. I have also seen this issue occur on sites other than Lichess however. For example, I have occasionally run into this issue when performing a search through the URL bar with DuckDuckGo as the search engine. I don't recommend trying to reproduce the issue that way though both because that happens far more rarely and because it may cause you to run non-free JavaScript while debugging the issue.
