Hi, is this a built in Image Viewer or a Stand Alone program ? When installed if was a stand alone did it not have a option to make it the default viewer per file format or is there a control for that in it's interface ?

Windows user here, is there not a Right Click option ?

Only application choice to open JPEG is always ask (which it does not do), save file, Firefox default, or other. I want to use Image Viewer as default. JPG. Opening JPEG in browser interferes with what I want to do because screen is covered with JPEG. This means I have to use chrome. Using Ubuntu 16.04

In MS Windows explorer, go to a folder with a jpeg image within it. Right click an image and select from the menu; Open With and select the "Choose default program" option. Now select Windows Photo Viewer or image and then click OK.

At this point when you double click a jpeg file, it will open in Windows Photo Viewer or Image Viewer depending on your system.

JPEG, PNG and GIF are image formats Firefox will always display in a tab unless instructed by the website sending the file to treat it as a download (either by sending a binary content-type header or by sending a content-disposition: attachment header). Possibly an add-on could work around this; add-ons can intercept your requests and modify the headers before Firefox sees them.