the UA pattern I've always seen looks like this: Roku/DVP-8.0 (048.00E04143A)
Where the numbers following DVP are the firmware version.
Mozilla is almost certainly not going to be in a default Roku user agent, since the device has no browser. Of course someone can set any value they want for the user agent in brightscript, though.
You are right in that a user-agent can be spoofed but as a web developer a user-agent is normally used to present content IE CSS based on their browsers capabilities.
You may have seen code based on this. For Chrome but Mostly Safari;
-webkit for Firefox,
-moz for Opera,
Opera didnt even use a user-agent for quite some time and started using Mozilla/5.0 to avoid legacy server rules.
Then you have the bots such as Googlebot/2.1. Which one can write codes based on allowing indexing to specific bots or disallow them all together..
Email clients and many others apps have user-agents which are not browsers. Basically any communication between server/clients over http use user-agents and it is mostly for content and semantic reasons.
Spoofing your user-agent is usually used for the reasons not intended.
And with that Roku does indeed use Mozilla/5.0
as well as ..Mozilla/5.0 (QSP; Roku; UI; 188.8.131.52) Mozilla/5.0 (compatible; U; NETFLIX)Roku 3/7.0 (Roku, 4200X, Wireless) Roku/DVP-4.1Roku/DVP-5.0Roku/DVP-7.0.