Mines would be no problem when it comes to de-orbiting - they're small enough to burn up completely, especially since the explosive charge would (presumably) ignite long before they could do any harm. However, they are certainly not a solution, because it would take far too many mines.
A better alternative would be to mine the jump points. This too, however, is not an option. Remember, all those Kilrathi fleets entered the system so quickly that Confed barely had time to react. There was no time whatsoever for defense - a retreat followed by a counter-offensive was not just the only sensible option, but actually the only option.
All in all, it is very difficult to fault Confed strategy during Firekka. The situation was that the entire Kilrathi fleet was in the Firekka system. Confed could have done the same. This, however, would have triggered a confrontation bigger than even MacAuliffe, but with us at a disadvantage, since they would be in defensive positions. This battle would have ended the Kilrathi War - but the risk of the wrong ending was too high. And, if we reject the 'entire fleet' option, Confed's 'disrupt the ceremony while advancing into Kilrathi space elsewhere' tactic was the only way to go. Even for the Firekkans, this was the healthiest possible option.