The code for the demo which provides 7 different sign in methods uses the FirebaseUI and is remarkably simple, see below.

library(shiny)
library(firebase)

ui <- fluidPage(
  useFirebase(),
  useFirebaseUI(),
  reqSignin(actionButton("signout", "Sign out")),
  uiOutput("msg"),
  plotOutput("plot")
)

server <- function(input, output){
  f <- FirebaseUI$
    new("session")$
    set_providers(
      email = TRUE,
      yahoo = TRUE,
      google = TRUE,
      github = TRUE,
      twitter = TRUE,
      facebook = TRUE,
      microsoft = TRUE
    )$
    launch()

  output$plot <- renderPlot({
    f$req_sign_in() # require sign in
    plot(cars)
  })

  output$msg <- renderUI({
    f$req_sign_in() # require sign in

    user <- f$get_signed_in() # get logged in user info
    print(user)
    
    h4("Welcome,", user$response$displayName)
  })

  observeEvent(input$signout, {
    f$sign_out()
  })

}

shinyApp(ui, server)