Thanks. Btw there's a chrome version in description, I made it recently.

"уже сохранённые делает приглушённым цветом"
Это можно. Под опцию. Сорт оф. Бегать по всей странице и перекрашивать все картинки я точно не буду, а вот под курсором чтобы картинка или даже сама кнопка как-то окрашивалась, если уже сохранялась, это можно сделать.

upd.
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I've added new option, you can completely disable spacebar hotkey now.

Maybe I messed up with how these folders are saved in settings, had to redo it few times.
The easiest would probably be to completely uninstall the extension, so it would delete all settings, and then installing it again. Or try "Reset all" button, maybe even that would be enough.
I'll re-check everything, maybe missed some bug. Thanks for the feedback.

Thanks.
Image Toolbar was my inspiration, I wanted same "Save" button but without everything else and with some control over the save process.

The button already blinks red if you saved already existing file.
Unfortunately I can't say that _before_ the download, WebExtensions don't have access to your files. Sorry, there's nothing I can do about it.

I'd like to do that, but it's impossible with current Downloads API. I can't save files to any folder outside of your default save folder, and I can't open "Save As" dialog in your last saved folder because it might be outside of it as well.
And even IF we're talking only about the folders inside the default save path, which there's no way of knowing, it would only be possible with some really dirty hacks.

https://bugzilla.mozilla.org/show_bug.cgi?id=1342563
They're talking about possible "useLastDir" option, that's exactly what you want. I'll add this feature the moment Mozilla implement it.
Thanks for the feedback.

Really sorry about that, just uploaded the new version, it should work again. Thanks for letting me know.

Thanks.
That's how it works now. Firefox Extensions don't have access to your local files. If you saved the file recently, I can be sure if it's a duplicate, but for your old files, the most I can say is if the file with this filename already existed, which is not enough, because some pages might have different pictures with the same filename, something like "image.jpg". Or some "1.jpg" file would prevent you from downloading another completely different file with this name.

Are you talking about the case when "Save as" dialog is displayed on save, i.e. opened folder always should be the last one you saved images to?
Iirc Firefox already remembers last folder, but only within each website. I'm gonna look at Download API, if there's an easy way to get last download location, then I'll add this option.

It still happens sometimes. Please give me an example and I'll try to fix it.

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Thanks for the link, I'll fix that in the next version.