Have you enabled minminguard in xposed?
I'm already using it with adaway since I discovered it doesn't block ads in firefox but it's a little bit misleading since you have config options for firefox in the minminguard.
Whats version did you check? I test MMG 2.0.9 and bootloop in Pie (xiaomi mi a2); edxposed 0.4.5.1 SandHook Riru 19.4 too.
2.09 is the last one i think.
Try using YAHFA-v0.4.5.1_beta.4463