There are two possible issues:
1) If you've got IHG from Firefox add-ons
– it's out of date! You need the current version, many solutions won't work without it. There are two parts to the IHG addon, the IHG code and the hostf – the hostf contains the solutions enabling IHG to work. The hostf gets updated / added to regularly. Get the current version of IHG here: http://hot-honeys.com/index.php?topic=3210.0
The most recent hostf will be installed the first time you use IHG.
2) FireFox is changing:
If you're using FF43 or 44 you can make IHG work but from about 45 onwards all add-ons will need signing.
IHG will not be signed.
Firefox will kill IHG soon anyway so it's not worth while getting it signed.
There is a lot of discussion about Firefox and it's change of model elsewhere on the forum (see the archives). The best advice for IHG users is quite simple -
FireFox and Pale Moon work fine when installed together.
They also work without problems if they're both open at the same time.