90 minute rating of a UL Class 350 safe means that the interior of the safe will stay below the autoignition temperature of paper when directly exposed to fire. So paper inside will not be charred, ASSUMING nothing else inside is susceptible to the elevated temperatures.
I'd expect thermally printed recepits for example to come out black. Plastic would melt. Digital media would likely be destroyed.
But if you're just looking to keep paper documents in readable (though they will be pretty well steamed) condition, you should be fine.
^^This! AND the previous advice about keeping digital copies of all important documents. Use the 3-2-1 Backup strategy (
https://www.backblaze.com/blog/the-3-2-1-backup-strategy/). Fire safes are designed to release water (steam) into the safe as part of the preservation for documents. You can put documents in a plastic bag to protect them from water/steam, but check the melting temp of the plastic bags because if they melt, the document could also be compromised that way.
For documents and media preservation, if your safe is big enough, put your documents inside a plastic bag, or if you're REALLY serious, a document protection bag like this one:
https://www.amazon.com/dp/B072C7HYZG/?tag=atomicindus08-20
Then put that in document safe like this one:
https://www.amazon.com/dp/B00GE586CY/?tag=atomicindus08-20
And if your Liberty Franklin is big enough, put that inside your Liberty Franklin.
Also, Never open a hot safe. The contents inside may be above the ignition temp but are preserved because of the lack of oxygen. Open the safe and ****, they will auto-ignite and you can watch your documents that survived the fire inside the safe, burn before your eyes.
It's very informative to google "opening a safe after a fire" - check out the pics and some YouTube videos.