I've used Omniwallet always with Chrome, if I remember well that is the preferred browser. Furthermore, there's a message on their homepage now stating:
"Backend Daemon Issues. We experienced a hiccup with one of our backend daemons.
It has been corrected and the client is restarting and rescanning the blockchain data
Until complete this will affect our ability to create any transactions and get new transaction info from the blockchain."
That explains it all, I think. Hopefully they are back in business soon.
Edit one hour ago this was posted on the Safe Forums:
"Hey guys. This is Adam at Omni. Our backend client on Omniwallet seems to have hit a hiccup over night. I've kicked it and its restarting/rescanning. You can login to omniwallet now however the backend is lagging behind (last block parsed 424584). Once the backend client finishes its resync/scan we'll get caught up pretty quick. Should take about 1-2 hours from current estimates "Still scanning.. at block 337126 of 424584.\nProgress: 26.96 % (about 75:32 minutes remaining)"}
I've posted a notice on the main website and will update /remove that when its done"