@Grimwear,
@crazybars,
@Anthony,
@DarkFenix,
@Cinsyac,
@maindish,
@jhs_reika
OK, found the solution for Firefox version 56 and older: install AbortController polyfill manually for your browser only via TaperMonkey add-on. Step-by-step instruction:
[ol]
[*]Download and install TamperMonkey version
4.11.6120 add-on for Firefox. You can download it from the
official Mozilla add-ons website. Scroll down to "Older versions" and click "+ Add to Firefox" button.
[*]Open
this gist, select and copy entire code to the clipboard.
[*]Click the TamperMonkey button on the browser toolbar. Select "Create a new script..." from the pop-up menu.
[*]In the editor you see select all lines, then delete them and paste the code from the clipboard.
[*]Right above the first line in the editor (i.e. above // ==UserScript==) click "File->Save".
Do not try to go to the browser's menu "File->Save". Click the one inside the page!
[*]Open new tab and check any manga on MangaDex. It should now show images.
[/ol]
I swear I didn't change anything from the AbortPolyfill I installed via npm. I only copied the code as is and modified the "// ==UserScript==" section. If you want to be sure that this script won't steal your passwords / credit card information / other valuable data, take the code from the official sources (link above) create new TamperMonkey user script and copy-paste it manually. Also, you can read and inspect the code (JavaScript knowledge might be required).
Also, you might change the
line to match MangaDex domains (I don't know which ones, don't ask me). Then AbortController polyfill won't work on other websites (and they'll still be broken), but at least you won't have to worry that it might steal your personal information from those websites.