Проблему решил плагином
https://github.com/wltb/ff_feedcleaner
Правило для плагина:
Код:
[
{
"URL":"forum.ispsystem.ru",
"type":"regex",
"pattern":"#(<link>(https:\/\/forum\\.ispsystem\\.ru\/showthread\\.php\\?)(\\d{0,})-(.*)</link>)#",
"replacement":"<link>showthread.php?$3</link>"
},
{
"URL":"forum.ispsystem.ru",
"type":"regex",
"pattern":"#(<guid isPermaLink=\"true\">(https:\/\/forum\\.ispsystem\\.ru\/showthread\\.php\\?)(\\d{0,})-(.*)</guid>)#",
"replacement":"<guid isPermaLink=\"true\">showthread.php?$3</guid>"
}
]