Is it safe to install Adblock plus on firefox?
Yes, the add-on is 100% safe.
Even if it was malicious, Firefox's add-ons are sandboxed so they have no interaction with your operating system.
Adblock Plus has a very large userbase (I use it too) so that is another factor you have to look at.
Unless you are installing it on a work computer and there is a policy put in place saying you aren't allowed, it's all good. The only issue you may run across is that some websites require you to see their ads in order to see their content or for them to function correctly.