In the old Crash reporting - there is easy way to upload. i didn't find it in the Crashlytics section
UPDATE: Firebase update their docs: see here
There is no need anymore to manually upload mapping file.
1 - To preserve the info Crashlytics needs for readable crash reports, add the following lines to your config file:
-keepattributes *Annotation*
-keepattributes SourceFile,LineNumberTable
-keep public class * extends java.lang.Exception
2 - To let Crashlytics automatically upload the ProGuard or DexGuard mapping file, remove this line from the config file (app/proguard-rules.pro):
-printmapping mapping.txt
For faster builds with ProGuard, exclude Crashlytics. Add the following lines to your config file:
-keep class com.crashlytics.** { *; }
-dontwarn com.crashlytics.**