Issue voting buttons look ugly on Firefox 52.9
### Summary
Icons on the buttons are huge.
### Steps to reproduce
1. Open Firefox 52.9.0 ESR on Linux X11;
2. Go to some issue on gitlab.com
3. Look at the buttons
### Example Project
https://gitlab.com/sultanahamer/OpenContacts/issues/85
### Possible fixes
Revert some incompatibe design changes. The problem started happening relatively recently.
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