This Firebase helper class uses transient Firebase objects, so that each object is never used for more than a single transaction. It also prevents Firebase crashes by creating mutable deep copies. The observe method also does sanity checking.

SNFirebase.h

//
//  SNFirebase.h
//  SharedNotes
//
//  Created by Zane Claes on 4/12/14.
//  Copyright (c) 2014 inZania LLC. All rights reserved.
//

#import <Foundation/Foundation.h>
#import <Firebase/Firebase.h>

@interface SNFirebase : NSObject

@property (nonatomic, strong) NSMutableDictionary *contents;
@property (nonatomic, strong) NSMutableArray *observers;
@property (nonatomic, readonly) Firebase *firebase;

- (void)save;
- (void)save:(void (^)(BOOL success))block;
- (void)load:(void (^)(BOOL success))block;

- (Firebase*)observe:(NSString*)path eventType:(FEventType)type expectedClass:(Class)cls withBlock:(void (^)(id copiedValue))block;
- (void)stopObservingChanges; // Removes all observers
- (void)startObservingChanges; // Needs to be subclassed

- (id)initWithFirebase:(Firebase*)firebase contents:(NSDictionary*)contents;

@end

SNFirebase.m

//
//  SNFirebase.m
//  SharedNotes
//
//  Created by Zane Claes on 4/12/14.
//  Copyright (c) 2014 inZania LLC. All rights reserved.
//

#import "SNFirebase.h"

@implementation NSDictionary (DeepCopy)

- (NSMutableDictionary *)mutableDeepCopy
{
  return (NSMutableDictionary *)CFBridgingRelease(CFPropertyListCreateDeepCopy(kCFAllocatorDefault, (CFDictionaryRef)self, kCFPropertyListMutableContainers));
}

@end

@implementation NSArray (DeepCopy)

- (NSMutableArray *)mutableDeepCopy
{
  return (NSMutableArray *)CFBridgingRelease(CFPropertyListCreateDeepCopy(kCFAllocatorDefault, (CFArrayRef)self, kCFPropertyListMutableContainers));
}

@end

@interface SNFirebase ()
@property (nonatomic, strong) NSString *firebaseURL;
@end

@implementation SNFirebase

- (Firebase*)observe:(NSString*)path eventType:(FEventType)type expectedClass:(Class)cls withBlock:(void (^)(id copiedValue))block {
  __block Firebase *fb = path.length ? [self.firebase childByAppendingPath:path] : self.firebase;
  [fb observeEventType:type withBlock:^(FDataSnapshot *snapshot) {
    if(![snapshot.value isKindOfClass:cls]) {
      NSLog(@"Object changed at %@ is not a %@; it is a %@ (%@)",snapshot.ref.description,cls,[snapshot.value class],snapshot.value);
      return;
    }
    id copied = nil;
    if(cls == [NSDictionary class] || [cls isSubclassOfClass:[NSDictionary class]] ||
       cls == [NSArray class] || [cls isSubclassOfClass:[NSArray class]]) {
      copied = [snapshot.value mutableDeepCopy];
    }
    else {
      copied = [snapshot.value mutableCopy];
    }
    NSAssert([copied isKindOfClass:cls],@"%@ was not a %@ after copying (%@)",copied,cls,[copied class]);
    block(copied);
  }];
  self.observers = self.observers ?: [NSMutableArray new];
  [self.observers addObject:fb];
  return fb;
}

- (void)startObservingChanges {
  NSAssert(NO,@"startObservingChanges should be overwritten");
}

- (void)stopObservingChanges {
  for(Firebase *observer in self.observers) {
    [observer removeAllObservers];
  }
  self.observers = nil;
}
- (void)save:(void (^)(BOOL success))completionHandler {
  Firebase *fb = [[Firebase alloc] initWithUrl:self.firebaseURL];
  [fb setValue:[self.contents mutableDeepCopy]  withCompletionBlock:^(NSError *error, Firebase *ref) {
    if(completionHandler) {
      completionHandler(error ? NO : YES);
    }
  }];
}

- (void)save {
  [self save:nil];
}

- (void)load:(void (^)(BOOL success))block {
  Firebase *fb = [[Firebase alloc] initWithUrl:self.firebaseURL];
  [fb observeSingleEventOfType:FEventTypeValue withBlock:^(FDataSnapshot *snapshot) {
    _contents = [[snapshot.value isKindOfClass:[NSDictionary class]]?snapshot.value:@{} mutableDeepCopy];
    block(_contents.allKeys.count > 0);
  }];
}

- (Firebase*)firebase {
  return [[Firebase alloc] initWithUrl:self.firebaseURL];
}

- (id)initWithFirebase:(Firebase*)firebase contents:(NSDictionary*)contents {
  if((self = [super init])) {
    _firebaseURL = firebase.description;
    _contents = [contents?:@{} mutableDeepCopy];
  }
  return self;
}

@end