To summarize the answer: switching to proprietary drivers should work, but the artifacts disappeared for me even on the free drivers now.
Update: since switching back to the proprietary AMD driver (fglrx), I haven't seen these artifacts.
Occasionally I see artifacts instead of correctly rendered characters:
For now I have only observed this in Firefox.
The characters become normal once I select the text.
What's the issue here and how can I solve it?
Graphics card: AMD Radeon HD 7790
Driver: radeon (free) -> now I'm no fglrx and am not seeing the artifacts (see the update)