Howto apply certain rules only at startup

[Johannes Sauer]

Hi,

what I want to do is automatically start application on a certain tag.
However when I later start another instance of the application I want it to
appear on the tag I am at that moment.
I tried to do something like this in rc.lua:

   rules_always  = {
       -- All clients will match this rule.
       { rule = { },
         properties = { border_width = beautiful.border_width,
                        border_color = beautiful.border_normal,
                        focus = true,
                        keys = clientkeys,
                        buttons = clientbuttons } },

       -- Set Thunderbird to always map on tags number 3 of screen 1.
       { rule = { class = "Thunderbird" },
         properties = { tag = tags[1][3] } },
   }

   rules_startup = {
       -- Set Firefox to always map on tags number 3 of screen 1.
       { rule = { class = "Firefox" },
         properties = { tag = tags[1][2] } },
   }

   awful.rules.rules = awful.util.table.join(rules_always, rules_startup)

then at the end of rc.lua:

   run_once("firefox")

   awful.rules.rules = rules_startup

However the effect is that still all rules will be applied, i.e. every new
instance of firefox will appear on tag 2.

What am I doing wrong?

Best,
Johannes

PS: My awesome version is 3.4.11, don't think it's relevant for this
question.