I have this same exact problem and I am not a Windows insider.

Which problem, exactly? What version of Zotero are you running?

If you're running Zotero 5.0 and it's bringing Firefox to the front, install the Zotero Connector in Firefox (to replace Zotero for Firefox) and uninstall the Zotero Word Integration plugin from Firefox.

Update:
Doesn't work. Using the newest version of Word-16. Zotero 5.whatever is the newest, just downloaded it to my D drive (My c drive is a small SSD). Zotero word for windows integration is enabled in extensions on add-ons manager. Opened word, clicked "add/edit citation" and "FF safe mode, FF closed unexpectedly etc..." pops up. I can see the new icons in word but they do absolutely nothing after I close the FF error window.

I think that's all the details.

I am using Microsoft Office 365 Proplus Version 1712 Build 8827.2148

The FF error is the same as this one: http://www.f15ijp.com/wp-content/uploads/2016/02/firefox-safe-mode.png

Thanks for the tip. I removed the old FF plugin from FF... I'm even considering uninstalling it. But when I click on add/edit citation (every third or fourth time) I get the FF error message. There must be some old FF something in the Zotero folders. If anyone knows of files associated w/ FF that may be in the Zotero folders, please let me know. I did do any uninstall, but didn't delete personal settings files (didn't want to have to redownload the library and everything). So maybe some old ext is in there somewhere.

I'll keep at it. Thanks for your help.

So the full story is, I was using word on an old (like 1 yr old, not that old) document and tried to edit the citations. It didn't work and I was wondering if I moved it or copied it and broke that documents connection to zotero. I only use zotero standalone typically for creating bibliographies and in text citations. I typically use the chrome plugin to create the reference in Zotero in the first place, but sometimes I'd use FF since it created better references.

So anyway, I went online and saw a post that said, version 5 fixes everything. So I downloaded it. Tried to use it and the buttons do nothing. Then I thought, inaccurately- I have to open FF. I'd forgotten that I never needed to before. So I opened it, and silly me, updated it. I closed everything down and started from fresh, with a restarted PC multiple times. I'd hit the add citation button in Word with FF either open or not open and I'd get that error. Then last night I uninstalled FF.

Now, after a fresh restart of my pc and no more FF on my pc at all, I no longer get that error. But clicking the zotero buttons in word does nothing (while zotero 5 is open). I'm not reinstalling word because I have text aloud plugins and student permissions and settings I don't need to mess up.

Can't figure out why zotero 5.0 does not work in Word. Otherwise, it works perfectly fine on its own as an indexer and a one by one reference maker. But I don't need an indexer. Tbh I use mendeley as an indexer. But I love zoteros bibliography and in text features while in Word. I was even considering get a sub and trying to tweet the indexing features now that mend has already created a folder database.

I have tried all the suggestions... making sure the extensions files are in startup, gone to tools in zotero and hit reinstall under the word processors section. I always start zotero as administrator. I could uninstall and delete every last file on my pc related to zotero, but my zotero library feeds my mendeley library. Only thing I can think of, its on D drive. Never was a problem before, if I recall, but maybe w/ 5.0 this is an issue. Maybe I'll try installing it on my Cdrive, but its almost full and I will have zotero folders strewn all across my hard drives (i already do from numerous copy and paste of the ext files and reinstalls). So thats all I got. GL everyone else!

@Nowshin2020: I'm not sure what you mean by "rebuilding the index". The full-text in the Zotero preferences? This wouldn't have anything whatsoever to do with that or with Better BibTeX. I assure you those things were not what fixed this.