Suggestion:
Batteries are made in different GROUPS
The firebird looks to be a group 75
The forester is a 40 amp hour batter looks to be a group 5
THey won't fit but the cranking amps looks like it would work but for 50 bucks you can go to wal-mart and be done with it.
{ 2 comments… read them below or add one }
Technically they should work as long as the terminals are the same (that is to say top mount or side mount). However if the battery is smaller (dimension wise) the car may eventually crank longer and slower when you try to start it due to insufficient requirements. If it is larger or the same size then there should be no problems.
If it fits, and the terminals are useable with it, — go ahead! As long as the "new battery isn't bigger than the "hole" it fits in, – you are ok, if it has different kind of terminals, adapters are available (don't cut off present terminals and put "clamp" on ones),– they are invitation to corrosion and connection probelms down the road!
99.9% of all cars on road recently have 12 volt systems, – only possibility would be that you have to get polarity right, and if "terminals" swapped (positions)–, – you might have problems making wires reach! And maybe 5% have positive ground, – and not negagtive ground, — but hooking right terminals up takes care of this! Lots of engilish cars in years past were that way, but they have "come around" to the standard of the rest of the world!