I can't actually seem to replicate the issue, sorry! I tried under Waterfox Classic 2021.02 as you mentioned, and it appears to work fine for me. Would you mind sending your settings (through the options->export button)? Contact links are available on the extension page (use whatever you're more comfortable with). Also, could you send an example link that doesn't work for you? For me, https://pbs.twimg.com/media/DlOC3mOWsAEi1ld.jpg redirects properly to https://pbs.twimg.com/media/DlOC3mOWsAEi1ld?format=jpg&name=orig .
It could be that there's a specific combination of settings that causes it to break, it wouldn't be the first time that has happened under Waterfox (due to its older JS engine).