Well, I have also tried to install in two other computers that I have at home, one of them running Windows and the other running Elementary OS. Nevertheless in both cases I got the same error message. Sincerelly I do not have any idea of what may be causing the problem, once I do not use antivirus or any other software which could interfer in the instalation proccess.
I have also tried to install in the mobile version (for Android), but without success too.
Here is the error log of browser console:
1485659743297 addons.xpi WARN Failed to remove temporary file C:\Users\Mauro\AppData\Local\Temp\tmp-kiw.xpi for addon https://addons.mozilla.org/firefox/downloads/latest/safesearch_enforcer/addon-775516-latest.xpi?src=dp-btn-primary: [Exception... "Component returned failure code: 0x80520015 (NS_ERROR_FILE_ACCESS_DENIED) [nsIFile.remove]" nsresult: "0x80520015 (NS_ERROR_FILE_ACCESS_DENIED)" location: "JS frame :: resource://gre/modules/addons/XPIProvider.jsm :: AddonInstall.prototype.removeTemporaryFile :: line 5682" data: no] Stack trace: AddonInstall.prototype.removeTemporaryFile()@resource://gre/modules/addons/XPIProvider.jsm:5682 < AddonInstall.prototype.onStopRequest/<()@resource://gre/modules/addons/XPIProvider.jsm:6179 < Handler.prototype.process()@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:940 < this.PromiseWalker.walkerLoop()@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:816 < this.PromiseWalker.scheduleWalkerLoop/<()@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:750
1485659743298 addons.xpi WARN Download of https://addons.mozilla.org/firefox/downloads/latest/safesearch_enforcer/addon-775516-latest.xpi?src=dp-btn-primary failed: [Exception... "Component returned failure code: 0x80520006 (NS_ERROR_FILE_TARGET_DOES_NOT_EXIST) [nsIZipReader.getEntry]" nsresult: "0x80520006 (NS_ERROR_FILE_TARGET_DOES_NOT_EXIST)" location: "JS frame :: resource://gre/modules/addons/XPIProvider.jsm :: loadManifestFromZipReader< :: line 1500" data: no] Stack trace: loadManifestFromZipReader<()@resource://gre/modules/addons/XPIProvider.jsm:1500 < TaskImpl_run()@resource://gre/modules/Task.jsm:319 < Handler.prototype.process()@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:937 < this.PromiseWalker.walkerLoop()@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:816 < this.PromiseWalker.scheduleWalkerLoop/<()@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:750
1485659743309 addons.xpi WARN Failed to remove temporary file C:\Users\Mauro\AppData\Local\Temp\tmp-kiw.xpi for addon https://addons.mozilla.org/firefox/downloads/latest/safesearch_enforcer/addon-775516-latest.xpi?src=dp-btn-primary: [Exception... "Component returned failure code: 0x80520015 (NS_ERROR_FILE_ACCESS_DENIED) [nsIFile.remove]" nsresult: "0x80520015 (NS_ERROR_FILE_ACCESS_DENIED)" location: "JS frame :: resource://gre/modules/addons/XPIProvider.jsm :: AddonInstall.prototype.removeTemporaryFile :: line 5682" data: no] Stack trace: AddonInstall.prototype.removeTemporaryFile()@resource://gre/modules/addons/XPIProvider.jsm:5682 < AddonInstall.prototype.downloadFailed()@resource://gre/modules/addons/XPIProvider.jsm:6217 < AddonInstall.prototype.onStopRequest/<()@resource://gre/modules/addons/XPIProvider.jsm:6180 < Handler.prototype.process()@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:940 < this.PromiseWalker.walkerLoop()@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:816 < this.PromiseWalker.scheduleWalkerLoop/<()@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:750
I have tried to update the version with some adjusts recommended by AMO team, but without success.
I strongly believe that It cannot be a simple coincidence (in four different devices). If you could also just test it for me, I would be glad.
Since now, thank you a lot.