The icons you have indicated are called, from left to right,
document.revert.xxx) where xxx would be the image format (typically
svg). I'm assuming the icons are named according to this.
What you have to do is to locate where these icons are: at a system-wide level they would be in
/usr/share/icons/name_of_icon_theme/. At the user level, they will be in
~/.icons/name_of_icon_theme if you have created such a folder. Within the particular folder, you may need to look in the subfolder called actions. Within actions, the icons maybe in
/24, etc. (At least one theme has the size first and then actions as a subfolder.) For me, the icons are spread over /16 and /22.
Depending on whether the icons you wish to replace are in
/usr/share/icons or in
~/.icons you will need to use sudo. First, rename the icons you think you want to replace, then copy over the icons from the source you desire. You need to make sure they're the same size (16, 22, 24, whatever) and the same format. And ignore the ones represented by logical links.
To see if the changes are indeed the ones you want, you need to change to another icon theme and then back to your desired theme to effect the changes (using something like Ubuntu Tweaks or MyUnity or Gnome Tweak tool). Alternatively, you can log out and log in.
So long as you're systematic, nothing will go wrong and you can always revert the changes.
Also, all apps that use these icons will be affected, not just Firefox!