Very slow Zoho Mail?

For the last week or so Zoho Mail seems to have starting functioning very slowly and having a few bugs.
It's slow to open mails, slow to send, slow to change between email accounts.
Sometimes clicking on a particular folder (eg Sent folder) stops working and I have to open a new browser tab for mail which works fine for a little while.
I also had an issue where all sent items appeared to have vanished until I started a new browser tab again, plus an instance where folders from a different email account were showing against my current mail account.
It used to work absolutely fine until a couple of weeks ago and I've not changed anything here.

I'm using Windows 7 64 bit and Firefox browser (26.0). I've tried clearing browser cache, rebooting etc but it's still the same. Is anyone else having the same issues or is it something local to me?

Many thanks,
Dave