Is that an HTML backup?

• https://support.mozilla.org/kb/how-do-i-use-bookmarks

• Bookmarks > Show All Bookmarks > Import & Backup > Import Bookmarks from HTML

no it isn't

as i said it is just files left from chrome install

{

  "checksum": "14499639c3..",
  "roots": {
     "bookmark_bar": {
        "children": [ {
           "children": [ {
              "date_added": "1304641..",
              "id": "8446",
              "name": "my-chrome.ru -- Google Chrome, news, flags",
              "type": "url",
              "url": "http://my-chrome.ru/"
           }, {
              "date_added": "129512516..",
              "id": "1686",
              "name": "chromeextensions.org -- Google Chrome Extensions",
              "sync_transaction_version": "3",
              "type": "url",
              "url": "http://www.chromeextensions.org/"

. . . .

OK, that is a Bookmarks file from Google Chrome. You would have to place this file in the default location that GC uses for its personal data and let Firefox import the GC bookmarks.

• https://support.mozilla.org/kb/Importing+bookmarks+from+Google+Chrome

Google Chrome stores the bookmarks in JSON format in the Bookmarks file in its profile folder. C:\Users\<username>\AppData\Local\Google\Chrome\User Data\Default\Bookmarks

thanks, are there simple windows-explorer-like actions in firefox bookmarks manager: like undelete last removed url/folder and cancel renaming?

If you can undo an action then Undo in the Organize menu in the Bookmarks Manager (Library) is enabled (not grayed).