Firefox - long img titles cut off
What you are seeing is the default behavior of Firefox. Rather than calling it a bug, I'd say that it is how Firefox for Android is designed. That said, I see that this is pretty inconvenient and you should probably open a bug report at bugzilla.
Meanwhile, you can use this handy addon (extension) for Firefox for Android which allows you to view the complete tooltip menu. It goes by the name Mobile Tooltip. You can also use this link to see it at Mozilla Addons (AMO). In your case, you can use it to read the full title tag of the image.