Firefox Focus is not a telephone app or a contacts app, so I think you're posting your question in the wrong place. Assuming you're on iPhone, try asking at https://discussions.apple.com/

Dear Chris, Many thanks for taking the trouble to respond to my query. Sorry if I didn't make my problem clear. I have a large address book in Thunderbird which includes many telephone numbers. The ordinary search panel in Thunderbird does not seem to recover telephone numbers and I wondered is there a way to find out if my address book contains a particular telephone number. Peter