

No, not in a new tab. I meant “display” as “it’s shown in the processed images list”. I upload the heic image, it gets read and compressed, and appears in the processed images list, as it should. But when I click the download button, I get the Firefox dialog to download a jpeg image, and if I proceed, I DO download a jpeg image. Regardless of the conversion format chosen.
(Other little quirk, if “Convert to” is set to JPEG, PNG, webP or ICO, the converted image will be shown in the processed list with the corresponding extension, but if set to Default, it’ll show as FILENAME. without extension)
Okay, here’s what I’ve found.
First of all, I use vanilla Firefox on windows 10, only extensions installed are AdBlockPlus and Privacy Badger, but even with those turned off and Firefox’s own Tracking Protection set to Standard instead of Strict doesn’t change the result. However, what I had missed is that ICO is the only conversion that works: it’ll take about a minute instead of the couple of seconds of the other cases, generate a file with huge size confronted with the original, but it will let you download an ICO file. Tried it on Edge, and every conversion works normally. The no-extension quirk for the Default option is still there, though.
This is what the Firefox console shows, from page load to the end of the conversion: