If "server_browser_default_filters" is set to "false", all SH servers are listed even if they have not responded to a ping or are incorrect versions. If it's set to "true", old servers and servers that do not respond to pings are filtered away (the default mode). So, if server browser appears empty, setting this to "false" might help.
"server_browser_refresh_interval" sets the delay between each ping that is sent to individual server. Increasing this number might help with some slow routers.
Please report if Firepower update and/or these additional settings help fix the issue.