Create the file user.js in your profile, and add this line,
user_pref("general.useragent.override","OS2zilla/5.0 (X11; Linux x86_64; rv:128.0) Gecko/20100101 SeaMonkey/2.53.23");
If you already have user.js, add the line. On startup the user agent will be set to OS2zilla, check in about:
Should work for Firefox too, untested.
Edit: Since we are using OS/2, I changed to this in user.js,
user_pref("general.useragent.override","OS2zilla/5.0 (OS/2; Warp 4.5 x86_64; rv:128.0) Gecko/20100101 SeaMonkey/2.53.23");