Something is wrong with Firefox's image auto resize
In forums, when I post an image, it's seems to be doing an incorrect image auto resize, for the image is still large to be fit in the websites' pages, and when I CTRL+Scroll, the browser makes a proper resize though making the letters and numbers smaller.
ప్రత్యుత్తరాలన్నీ (1)
Resizing images automatically only works if you open an image in a tab and there is only that image to be shown. In all other cases it is the website that determines how large an image is displayed.
If you post an image on a forum and the forum doesn't resize the image automatically then use an upload host that automatically generates a thumbnail that you can show and that opens the full sized image if you click the thumbnail.