I don't know if you fixed the problem, but I will post my solution in case someone else has the same problem.

Problem

I encountered the problem with Magento 2.3.5-p2 with PHP 7.3 and Firebear/ImportExport v3.1.6. I think that the main issue is that the extension versions below 3.2.0 are not fully compatible with Magento 2.3.2 onwards as it's stated in the extension changelog:

3.2.0 PHP 7.3 is fully supported. Besides, the extension is compatible with Magento 2.3.3. We’ve also added timestamp added for each log entry and a selector that allows removing product associated images from the server if the extension constantly imports new ones. All attributes are now included.

The full exception is:

Warning: Illegal string offset '__disableTmpl' in /vendor/magento/module-ui/Component/Form/Element/AbstractOptionsField.php on line 69 {"report_id":"8fda31ec65ff454e4ab0079d62b33273fb75cf30f01f640582de51ebf99835a8","exception":"[object] (Exception(code: 0): Warning: Illegal string offset '__disableTmpl' in /vendor/magento/module-ui/Component/Form/Element/AbstractOptionsField.php on line 69 at /vendor/magento/framework/App/ErrorHandler.php:61)"} []

The problem actually comes from Firebear\ImportExport\Ui\Component\Listing\Column\Duplicate\Options.php toOptionArray() method, because it's returning an array of strings and you can see that line 69 in AbstractOptionsField.php is trying to add a value to an array and thus throwing the Illegal string offset error.

AbstractOptionsField.php

public function prepare()
{
    $config = $this->getData('config');
    if (isset($this->options)) {
        if (!isset($config['options'])) {
            $config['options'] = [];
        }
        if ($this->options instanceof OptionSourceInterface) {
            $options = $this->options->toOptionArray();
        } else {
            $options = array_values($this->options);
        }
        if (empty($config['rawOptions'])) {
            $options = $this->convertOptionsValueToString($options);
        }

        array_walk(
            $options,
            function (&$item) {
*line 69*       $item['__disableTmpl'] = true;
            }
        );

        $config['options'] = array_values(array_replace_recursive($config['options'], $options));
    }
    $this->setData('config', (array)$config);
    parent::prepare();
}

Options.php

class Options implements OptionSourceInterface{

/**
 * @var array
 */
protected $options;

/**
 * @var array
 */
protected $duplicateFields = [
    'product' => ['sku', 'scope', 'url_key'],
    'customer' => [\Magento\CustomerImportExport\Model\Import\Customer::COLUMN_EMAIL],
    'address' => [],
    'composite' => [\Magento\CustomerImportExport\Model\Import\Customer::COLUMN_EMAIL],
    'cmsPage' => []
];

/**
 * Get options
 *
 * @return array
 */
public function toOptionArray()
{
    $newOptions = [];
    foreach ($this->duplicateFields as $fields) {
        $newOptions = array_merge($newOptions, $fields);
    }

    $this->options = array_unique($newOptions);

    return $this->options; // options looks like this -> ['sku', 'scope', 'url_key', 'email']
}
}

Solution

I fixed this by creating a preference for the Firebear\ImportExport\Ui\Component\Listing\Column\Duplicate\Option.php, overriding the toOptionArray() method, and returning the options in the required format.

Preference

class DuplicateOptions extends Options
{

/**
 * Get options
 *
 * @return array
 */
public function toOptionArray()
{
    $newOptions = [];

    foreach ($this->duplicateFields as $fields) {
        $newOptions = array_merge($newOptions, $fields);
    }

    $newOptions = array_unique($newOptions);
    $newOptions = $this->convertOptionsToRightFormat($newOptions);

    $this->options = $newOptions;
    return $this->options;
}
 /**
 * Converts the option array to the required format from AbstractOptionsField.php
 * @param array $array
 * @return array
 */
private function convertOptionsToRightFormat(array $array){
    for($i = 0; $i < count($array); $i++) {
        $array[$i] = ['label' => $array[$i], 'value' => $array[$i]];
    }
    return $array;
}
}

di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Firebear\ImportExport\Ui\Component\Listing\Column\Duplicate\Options" type="Vendor\Module\Model\DuplicateOptions"/>
</config>

Don't know if this is the cleanest solution, but it will work temporarily. The best one will be just to update the extension to version >= 3.2.0.