Did firestone align all four wheels, or just the fronts? Our cars have adjustments at all four corners, and if you have a worn upper control arm bushing (common for our age/mileage) or worn toe compensators it can cause issues.
I take my car to a guy who does local dirt track racing (and wins like crazy), he's very good, and very thorough, and even better, will align it to the "aggressive" specs on our page.
My car does not wander on flat road, nor on regular roads even if they're turtlebacked. Yours shouldn't either.
I suspect either they didn't align the rear, or you have a worn component that they didn't realize needs replaced.
97 Thunderbird LX (The GT Bird)
PI Swapped NPI motor with some port work on the heads, PI cams, 80mm MAF, CAI that feeds from fenderwell. Jmod, 3.73:1 TL in Mark VIII carrier, Mark VIII aluminum LCAs, 93 Mark VIII driveshaft, PBR brakes (soon to be Cobras), 18x9 wheels, 255/40/ZR18 Tires, Front and Rear strut/shock bracing, GR-2 shocks, Eibach 1.5" springs, 1989 SC front and rear sway bars.