Unfortunately, I'm not sure I completely understand the problem you're reporting with Firefox (nor do I know what an "integrar" or a "gar") is.
I'm not sure what you were trying to change or how you were trying to change it -- perhaps you are in the Firefox configurations page at "about:config"??
If so, there's a reason why users are shown a warning about messing with those settings -- changing them around without a good idea of what one is doing can seriously break Firefox.
So all I can offer is some general advice.
First, Firefox "out of the box" is plenty fast, especially on a modern system with good hardware. There's generally no need to perform complicated tweaks or fixes (e.g. from 3rd-party YT videos) to make it faster.
While it can be easily and extensively customized by both adjusting its built-in settings and by installing extensions (a.k.a. "add-ons") and themes, at some point, one risks the chance of creating performance issues by having too many add-ons, conflicting add-ons, outdated/unsupported add-ons, etc.
There is another overlay with "global" plug-ins (installed system-wide on the computer, rather than directly into Firefox), and with one's security applications (anti-virus, anti-malware, firewall, custom hosts file, etc.) They, too, can impact performance.
Most problems with Firefox originate in the user's profile, not in the program itself. So reinstalling Firefox will not always fix a profile problem, unless one deletes one's personal data/profile when one uninstalls the program. The user's profile needs to be either fixed or (if corruption is severe) replaced with a new one.
However, if you did make changes in "about:config", something may be broken there, too.
Without a good understanding of your exact issue or how it was created, it's hard to provide specific advice on how to fix it.
Here are two of the best resources for step-wise, logical troubleshooting. The steps range from merely diagnostic (such as Mozilla Safe Mode -- not to be confused with Windows Safe Mode) to more "invasive" (resetting Firefox to creating a new profile to a totally clean, from scratch reinstall + new profile).
I suggest working your way through these to try to isolate and (if possible) resolve your issue:
Also, for the record, to avoid getting into these sorts of predicaments, it's advisable to backup one's profile BEFORE making changes to it (installing new add-ons, changing settings, tweaking "about"config" settings, etc.) There are many ways to do this, both manually and using 3rd-party add-ons, such as FEBE.
That way, if something goes South, one can recover quickly.
So, if you can please clarify for us WHAT you were trying to accomplish, HOW you were trying to accomplish it, and WHAT problems you are now seeing (perhaps with a screen shot of the error message), that would help us to better help you.
Beyond that, I suggest trying the tutorials linked above.
Please let us know how it goes, so that we can all learn,