Very closely related questions have been asked before. Here are links to some of the answers:
I like this explanation:
Such messages are common with Gnome applications and do not necessarily indicate a bug. Gnome takes a dumbed-down approach to users; the intended audience won't run an application from a terminal and so won't see these messages. Redirect error output from Gnome applications to a log file if you really care, or to /dev/null otherwise.
And this is also nice:
This error indicates that the software is using perfectly valid GTK library calls with a newer version of the GTK library. Sometimes during software development it is desirable to get rid of the, "old way", to do things while maintaining backwards compatibility. This is accomplished by marking the undesired calls depreciated. This tells the developer that their code is perfectly valid in this release, but in the future their code will need to be updated to work with a new version of the library, possibly many releases down the road.
In this case, GTK wants to be more strict about including units on dimensions. It tells the developer that GTK is assuming the units are pixels, which is the exact assumption that the software developer made in the first place.
Source: Bailley S
By the way, if you launch Firefox from terminal you'll may see a terrifying message like this:
GLib-CRITICAL **: g_slice_set_config: assertion
But it seems to be harmless as per comment in Bug #833117.
In short, there's mostly no need for us beginners to launch GUI-based apps from the terminal unless there's a problem and we need to give output to someone who can understand the meaning of such output.