User (browser) agents in web scenarios are constantly out of date. They need to be updated more frequently. As of now there is still "Firefox 33.0 (Windows)" present, but in reality most people probably use version 64.0. In order for user not to wait while Zabbix updates them once in a while, the user could add his own user agents.
The configuration could be located in Administration->General. A new sub-section in dropdown, where only administrator can add them. Most likely administrator knows what he's doing.
The most basic set-up would require only two fields: "name" and "agent string" (label naming is up for discussion as well). Those are available online if anyone would wish to add his own specific version that works best.
Currently they are categorized by browser name. The categorization could be kept, but also it could be skipped and simply show a list sorted A-Z.
Several changes are required. Database requires a new table where to store the user agents. With ID, name and agent string field. If the categorization is kept, there could be one more column for category. User agents in dropdown are then grouped by matching category names. The ones that do not match and if there is more than one, could go to category "Other" ("Others" plural?) like it is now.
The built in "Zabbix" agent will remain there in the list and is build in the code, not in DB.
Some other user agents (that have only one version and have never ever changed) could be built-in or already predefined in that DB table. But what would mean that again Zabbix would need to update them once in a while, so that the list is not empty on fresh installation.