Certainly a fair remark, though just for clarity I'd like to point out this is simply the nature of twitter's image hosting/linking, not a shortcoming in the addon. File extensions are a mere semantic convention and mostly only relevant in filesystems for human users. In HTTP-hosted resources, they're a mere courtesy a server/URL can decide not to offer. Other websites will similarly not include a file extension, and there's little an addon or browser can do about it (especially in firefox, which still doesn't implement filename manipulation for addons).

I just checked again, and the file extension could be derived from other data in twitter's URLs, but it would require both the missing renaming implementation in firefox as well as special handling code in my addon just for twitter. The latter I indeed choose not to implement, even for the chrome-based browsers that would support it. :)

You may need to reload a page the first time after installing this addon. Double-clicking a regular image just works by itself (note that not everything that looks like a regular image on your screen will be detected as such, but the vast majority will be). The hover button also does not just show in the middle of the page in normal circumstances. but on an image. If this is your experience, you're testing it on just 1 website, and one that happens to do odd things this addon can't support with images. Please try it on other sites too before judging, or open a ticket on gitlab (or email me) and at least link me to the page so I can have a look.

Assuming the translation by google is correct: I won't build site-specific support into this tool. It's meant to be a generic image downloader that works well on sites with conventional HTML structures that expose images in an accessible way. Instagram is one of those sites with overly complicated, image-hiding webpage structures. I find a 2-star rating just for not working well on instagram a bit superficial since there's so much more to this addon and it works fine on most websites, but ok.