A big mess of pros and cons. Rated 3 out of 5 stars

So what it does, it does quite well. That is to help you monitor internet activity on your computer, at least if firefox is the only browser you make available. One add-on that both disables private browsing and prevents history from being deleted seems like a great package.
The downside is that there is no way to "cleanup" after I have checked the history. It builds up endlessly. I think this is the reason my firefox is now crashing more frequently then internet explorer ever did. If there was a simple way to turn this off and on than this wouldn't be a problem. The developers say it can be deleted by restarting in safe mode and running firefox with no add-ons. But I've tried this (on windows 7, firefox 19.0) and even in safe mode I cannot run firefox without add ons. There is just no option that suddenly appears by being in safe mode. Unfortunately, the only solution I can see is uninstalling and reinstalling firefox. And then I would end up reinstalling this anyway. What a hassle!
I hope the developers are reading this too. They say it is neccessary to prevent the add-on from being deleted, but this is not true. My add-ons are password protected by an add-on called public fox. If password protection is beyond their abilities, then perhaps they could reccomend Public Fox along with this.
With all that said, I do still use this add-on because even with all it's baggage it is still the best solution I've found. It's a shame that between Mozilla's Firefox, Google's Chrome, and Microsoft's Internet Explorer, none of them have been able to provide parental controls that actually work. So I settle for this. What a shame that there isn't a better solution then this third party add-on that makes the browser crash. I hope you read this too Mozilla. I expect better.

This review is for a previous version of the add-on (1.7.1). 

First off: a large history or using this add-on does not make Firefox crash. I never clean my history, and Firefox runs just fine. If you do not see the option to disable all add-ons then you did not start Firefox in Safe Mode. Note that starting Windows in Safe Mode has nothing to do with Firefox Safe Mode.

Password protecting the removal of this add-on and/or the deleting of the history is an interesting option I'll consider for a future version.