If the only reason for staying with 5.91 is due to a script that you are using, I would recommend contacting the authors of the script and enquiring of an update for the script.
Most scripts are compatible with more up-to-date versions of mIRC, as Khaled tries very hard to ensure that an update to the program is backwards compatible with previous versions.
The main exception to this, to my knowledge, is if there is a bug in the older version of mIRC that gets fixed.