XnView Software

correct.
in ubuntu xnviewMP; not talking about classic Windows version. try it yourself go to firefox cache folder in ubuntu, you can see the images, however in full view xnviewMP will not move forward or back within the images - in windows yes.

ubuntu is free so testing is easy.

xnview can not ...

in 'classic' under browser->file list->misc there is a check box - recognize only by extension. if box is unchecked program will show all graphic files even if they do not "a correct extension"

xnviewmp will also show graphic files but unlike 'classic' the user can not move back or forward between ...